Method and apparatus for transmitting data from a transmitter device to a plurality of receiver devices

ABSTRACT

Apparatus for communicating data between a transmitter device 16 and a first 51 and a second 52 receiver device, the receiver devices being connected, in use, to the transmitter device via a first 21 and a second 22 pair of wires respectively. Each receiver device is operable to receive signals detected as a change over time in the potential difference across the local ends of each respective pair of wires extending between the respective receiver device and the transmitter device, and the transmitter device is operable to transmit signals onto the wires at the transmitter ends thereof in order to transmit signals via a direct differential mode to each respective receiver via its respective pair of wires. The transmitter device is additionally operable to transmit signals to both receivers via a single common indirect channel and comprises: a channel estimator 1670, 1680, 1690 for estimating the extent of coupling between the common indirect channel and each of the receiver devices based on readings received by the transmitter device from the receiver devices; and a processor 1690 for determining a plurality of weighting values in dependence upon the estimated extents of the couplings; the transmitter 16 being operable to transmit a first signal via the direct differential mode over the first pair 21, to transmit a second signal via the direct differential mode over the second pair 22 and to transmit a combined signal onto the indirect channel, wherein the combined signal comprises a weighted sum of the first and second signals, the weighting being done in accordance with the determined weighting values; and wherein the transmitter further comprises a precoder 1640 for precoding the first, second and combined signals to pre-compensate them for the expected effects of cross-talk from the other ones of these signals, wherein the pre-coding of each signal, including the first and the second signals, is performed in dependence upon the determined weighting values.

This application is the U.S. national phase of International ApplicationNo. PCT/EP2016/054441 filed 2 Mar. 2016, which designated the U.S. andclaims priority to EP Patent Application No. 15275057.6 filed 2 Mar.2015, the entire contents of each of which are hereby incorporated byreference.

FIELD

The present invention relates to a method and apparatus for transmittingdata from a transmitter device to a plurality of receiver devices, andin particular to a method and apparatus for transmitting and receivingdata signals over pairs of wires. Such methods include all of thevarious Digital Subscriber Line (DSL) methods as specified in variousInternational Telecommunications Union (ITU) standards and as beingfurther developed in the ITU at present. Typically each such pair ofwires comprises a twisted metallic pair (usually copper) as commonlyfound within telephone access networks throughout the to world.

BACKGROUND

DSL technology takes advantage of the fact that although a legacytwisted metallic pair (which was originally installed to provide merelya Plain Old Telephone Services (POTS) telephony connection) might onlyhave been intended to carry signals using differential mode atfrequencies of up to a few Kilohertz, in fact such a line can oftenreliably carry signals at much greater frequencies. Moreover, theshorter the line, the greater is the range of frequencies over whichsignals can be reliably transmitted (especially with the use oftechnologies such as Discrete Multi-Tone (DMT), etc.). Thus as accessnetworks have evolved, telecommunications network providers haveexpanded their fibre optic infrastructure outwards towards the edges ofthe access network, making the lengths of the final portion of eachconnection to an end user subscriber (which is still typically providedby a metallic twisted pair) shorter and shorter giving, rise tocorrespondingly greater and greater bandwidth potential over theincreasingly short twisted metallic pair connections-without is havingto bear the expense of installing new optic fibre connections to eachsubscriber. However, a problem with using high frequency signals is thata phenomenon known as crosstalk can cause significant interferencereducing the effectiveness of lines to carry high bandwidth signals insituations where there is more than one metallic pair carrying similarhigh frequency signals in close proximity to one another. In simpleterms, the signals from one pair can “leak” onto a nearby line (whichmay be carrying similar signals) and appear as noise to the other line.Although cross talk is a known problem even at relatively lowfrequencies, the magnitude of this effect tends to increase withfrequency to the extent that at frequencies in excess of a few tens ofMegahertz (depending on the length of the lines in question), theindirect coupling (e.g. from a near end of a second line to a remote endof a first line) can be as great as the direct coupling (e.g. from thenear end of the first line to the remote end of the first line).

In order to alleviate the problems caused by cross talk (especially FarEnd Cross Talk or “FEXT” as it is known) a technology known as vectoringhas been developed in which knowledge of the signals sent overcrosstalking lines is used to reduce the effects of the crosstalk. In atypical situation a single DSLAM acts as a co-generator of multipledownstream signals over multiple cross-talking lines and also as aco-receiver of multiple upstream signals from the same multiplecross-talking lines, with each of the lines terminating at a singleCustomer Premises Equipment (CPE) modem such that no common processingis possible at the CPE ends of the lines. In such a case, downstreamsignals are pre-distorted to compensate for the expected effects of thecross-talking signals being sent over the neighbouring cross-talkinglines such that at reception at the CPE devices the received signals aresimilar to what would have been received had no cross-talking signalsbeen transmitted on the cross-talking lines. Upstream signals on theother hand are post-distorted (or detected in a manner equivalent totheir having been post-distorted) after being received at theco-receiver (the DSLAM) in order to account for the effects of thecross-talk which has leaked into the signals during their transmission.

WO02013026479 applied for by Ericsson proposes a method of transmittingsignals, in such a situation (i.e. where an indirect coupling iscomparable to a direct coupling for a given line), which involvestransmitting signals intended for reception by a single CPE device (afirst CPE device) onto both the line directly coupled to the first CPEdevice and onto a crosstalking line coupled only indirectly to the firstCPE device (it being directly coupled to a second CPE device). A TimeDivision Multiplexing (TDM) method is used to enable data to be sent (indifferent time slots) to the two respective CPE devices (with data beingsent over both wires at the same time to only one of the CPE devices ata time). In order to ensure that the two signals constructivelyinterfere at the receiving CPE device, the same signal as sent over oneline is pre-distorted (e.g. to introduce a delay and/or phase change)before being sent over the other to account for differences in thedirectly vs the indirectly coupled paths.

In addition, transmission mode uniqueness is not guaranteed whenmultiple conductors are in close proximity. In fact, it has beendemonstrated that multi-mode co-existence is inevitable inmulti-conductor environments. Intuitively, the average voltagepotentials of the pairs at a specific frequency are very unlikely to beequal. Due to this, the voltage potential between pairs starts to movein additional differential circuits formed from multiple pairs in asimilar fashion to those in twisted metallic wires pairs. Theseadditional modes/circuits are known as phantom modes. Additionally, itis possible for signals to travel over one or more wires with referenceto a fixed common ground (earthed) potential, and such modes arereferred to as common modes of transmission. The presence of additionalmodes, e.g. common/phantom or mixed modes, allows mode conversioncontinuously coupling signals (often destructively) in each mode. Unlikecrosstalk between pairs, signals over mode conversion crosstalk cannotbe corrected or controlled without a physical access to theseinterfering modes. Moreover, it is worth noting that phantom modespropagate over untwisted pairs. Hence, phantoms radiate (cross-couple)higher crosstalk levels than in ordinary pairs (which are twisted).Therefore, the differential mode suffers from energy dissipation underuncontrolled multi-mode channel environment especially at highfrequencies.

EP2091196 by Alcatel-Lucent provides a method to inject signals into thephantom mode formed between two Twisted Metallic Pairs (TMPs). Theinjected signals are the same as those sent onto one of the TMPs, butphase-rotated so that when converted and coupled into the differentialmode, they interfere constructively with the signals sent directly overthe respective one of the TMPs in the normal differential mode. However,EP2091196 does not consider how to exploit this technique in moregeneral circumstances where there is more than one possible phantom modeavailable (i.e. where there are more than two TMPs). Furthermore, EP2091196 does not address any power constraint implications of thearrangement.

WO2006/120510 describes a method for superimposing phantom-mode signalsonto multiple wire pair connections in order to reinforce existingdifferentially driven DSL downstream signals in a vectored binder ofDSLs. The method is carried out by treating each pair as a common-modeantenna with respect to earth ground, with some pairs selectivelyexcited at the transformer centre tap at the transmit end with respectto a common (earth or chassis) ground reference while correspondingreceivers on other non-excited pairs sense the signals between theircentre taps and a ground at the opposite ends of the lines to theexciting transmitters. A dual use with hybrid circuits allows thereceiving circuit to also have an upstream transmitter and anupstream-sensing receiver on the centre tap of the opposite side of anadjacent wire.

ITU-T: “DRAFT RECOMMENDATION ITU-T G.993.5 SELF-FEXT CANCELLATION(VECTORING) FOR USE WITH VDSL2 TRANSCEIVERS”, pages 1-80, 22 Apr. 2010,describes a single method of self-FEXT cancellation, in which FEXTgenerated by a group of near-end transceivers and interfering with thefar-end transceivers of that same group is cancelled by eitherpre-coding or post-coding to compensate for the expected effects ofcross talk from other transceivers in the same group using knowledge ofthe signals transmitted by the other transceivers in the group togetherwith measure knowledge of the cross-talk channel transfer functions.

SUMMARY

According to a first aspect of the present invention, there is provideda method of transmitting data from a transmitter device to a first and asecond receiver device, the receiver devices being connected to thetransmitter device via a first and a second pair of wires respectively,each receiver device being operable to receive signals detected as achange over time in the potential difference across the local ends ofeach respective pair of wires extending between the receiver and thetransmitter device, the transmitter device being operable to transmitsignals onto the wires extending between the transceiver device and thereceiver devices in order to transmit signals via a direct differentialmode to each respective receiver, and is additionally operable totransmit signals to both receivers via a single common indirect channel(which in some embodiments might be a phantom mode channel or else athird pair of wires with a non-negligible cross talk coupling to bothreceivers), the method comprising: measuring the extent of couplingbetween the common indirect channel and each of the receiver devices,determining a plurality of weighting values in dependence upon themeasured extent of the couplings, transmitting a first signal via thedirect differential mode over the first pair and a second signal via thedirect differential mode over the second pair and transmitting acombined signal onto the indirect channel, the combined signalcomprising a weighted sum of the first and second signals, the weightingbeing done in accordance with the determined weighting values, andwherein the first, second and combined signals are all pre-coded, priorto being transmitted, in order to pre-compensate for the expectedeffects of crosstalk coupling from the other signals, the pre-codingbeing performed using predetermined pre-coding coefficients, and whereinthe pre-coding coefficients for all three signals are calculated independence upon the determined weighting values.

In this way, it is possible for a single common indirect channel to beused to benefit both receivers simultaneously by using weighting valuesbetween 1 and 0. It is also possible to use different weighting valuesfor different tones. In this way the different extent of the couplingsat different frequencies can be taken into account and exploited tomaximise the total benefit to the two receivers (e.g. for tones where astronger coupling is in place for the first receiver compared to thesecond a bigger weighting can be given to the first signal (possiblyeven a weighting of 1 for say the first signal and zero for the secondsignal—implying that the combined signal for such tones is composedentirely of the first signal) whilst for tones where the coupling isstronger with the second receiver the weightings can be reversed to givea greater weighting to the second signal, etc.). Moreover, the weightingcan be determined in order to satisfy a number of differentobjectives—e.g. to provide greater assistance to a poorly performingline, or to maximise the total performance of both lines in combination,etc.

By performing the pre-coding in a way which takes into account whateverweighting values have been pre-determined (based on the extent of thecouplings—i.e. the magnitudes of the relevant channel transferfunctions—and the objective (which may be implicit but which ispreferably explicitly user specifiable) to be achieved—e.g. maximisingtotal capacity of the set of lines or ensuring at least a minimum levelof performance for both lines, etc.) the lines can pre-compensate forthe expected effects of cross talking from all other signals—includingthe combined signal (in particular the component of the signal which isnot benefitting the line whose signal is being pre-coded—e.g. signal 1on line 1 can be precoded to pre-compensate for the expected cross talkfrom the combined signal including a weighted component of signal 2 aswell as from the signal 2 itself from line 2).

Preferably, the weighting values are additionally determined independence upon the instantaneous level of demand for data to betransmitted to a respective receiver. It is most preferred if thetransmitter devices and the receiver devices are operating in accordancewith a physical layer retransmission scheme whereby a receiver requestsretransmission of received data which is irreparably damaged because oferrors in the received signals (and/or the detected and/or recovereddata upon receipt). In such a case, it is preferred if the demand usedin determining the weighting values reflects the demand for physicallayer re-transmission of data caused by such errors. In this way, higherlayers (e.g. data link, network, transport, application layer protocols)can be offered higher consistent data rates with less overhead (and lesspotential buffering) required for physical layer retransmissions (to bebuilt into the offered consistent rate to higher layers) because thebandwidth needed for these physical layer retransmissions can beallocated from the extra capacity associated with the use of the commonindirect channel on an on-demand basis.

Preferably the weighting values are re-determined on a relativelyfrequent basis such as of the order of between once every few seconds toseveral times per second. Most preferably, this is achieved bymonitoring relevant metrics such as channel transfer functionestimations and user demand levels (or estimations thereof) andre-determining the weighting values and resulting pre-codingcoefficients (possibly on a tone-by-tone basis especially where achannel transfer function estimation indicates a significant change onlyin respect of a subset of the tones in use) only if the monitoringindicates a change above a certain predetermined threshold has occurred.By careful setting of the size of such a threshold the rate at whichweighting values are re-determined can be optimised (to balance load onthe processor against the closeness with which changes in thechannels/demand are followed). This can enable changes in demand forbandwidth for the transmission of data to the different receivers to beaccommodated in a short period of time and consequently enables buffersat the transmitter associated with retransmission protocols to becleared to as quickly as possible. For example, physical layerretransmission performed in accordance with ITU recommendation G.998.4uses a queue of Data Transmission Units (DTUs) for retransmission whichis cleared as acknowledgements of receipt of DTUs are received at thetransmitter and weighting values could therefore be changed in preferredembodiments of the present invention in dependence upon the size ofthese queues in respect of the different receivers to which the DTU'sare destined.

Preferably the channel transfer functions are estimated on a regularbasis and the weighting values and/or precoding coefficients arerecalculated in the event that the channel transfer functions aredetermined to have changed by more than a predetermined amount since theweighting values and/or precoding coefficients were last calculated (orrecalculated).

As mentioned above, the weighting values are used not only to affect thesignal which is transmitted onto the common indirect channel, but arealso used to affect the first and second signals since the extent ofprecoding required to accommodate distortion caused by cross-talk variesin dependence upon these weighting values. In other words, it ispreferred if the first signal is generated in dependence upon (atleast): user data to be transmitted to the first receiver, channelestimations of the respective direct channel between the transmitter andthe first receiver, channel estimations of the indirect channel betweenthe direct channel between the transmitter and the second receiver onthe one hand and the first receiver on the other hand, channelestimations between the common indirect channel on the one hand and thefirst receiver on the other hand, and on the determined weightingvalues. The second signal is preferably similarly dependent uponcorresponding factors mutatis mutandis.

Preferably, the manner in which the weighting values are used to affectthe first and second signals (and any other signals for directtransmission to a predetermined receiver via a direct differential modechannel) and the manner in which the (or each) combined signal isgenerated for transmission over a common indirect channel (or multipleindirect channels) is performed in such way that by assigning zero toall of the weighting values for a particular tone, the method reverts tostandard vectoring (for any such tone) in which signals (the first andsecond signals) are transmitted only onto the direct differential modechannels, and those signals are pre-coded to pre-compensate for theexpected cross-talk effects of the signal being transmitted onto theother direct differential mode channel (or channels) associated with therespective other pair (or pairs) of wires.

It should be noted that embodiments of this aspect of the presentinvention permit more than two receivers to be coordinated within acoordinated group and moreover permit more than one common indirectchannel to be employed. One particularly advantageous embodimentinvolves the use of multiple different types of indirect channels to beused. In particular, some embodiments may use one or more phantom modechannels over a first set of tones and a combination of phantom modesand differential-mode crosstalk channels (e.g. channels where the normaldirect differential channel is not used directly at some tones) across asecond set of tones, wherein the second set of tones is generallysmaller than the first set. A particularly advantageous application ofsuch an embodiment is where one of the receivers is not capable ofreceiving tones above a certain cut-off frequency (perhaps because it isconfigured to operate in accordance with a DMT mode which only employstones up to a certain maximum frequency which is below the maximumfrequency which can be used in accordance with a different mode ofDSL—for example, many VDSL2 modems (using profiles 12 a or 12 b forexample) can use tones up to 12 MHz whereas most ADSL2+ modems only usetones up to 2.2 MHz); thus, in an example such case, a third receivermight be an ADSL2+ modem which is only configured to receive tones up to2.2 MHz whereas the first and second receivers could be VDSL2 modemsable to receive tones up to 12 MHz. In this case for tones in excess of2.2 MHz can be used for the benefit of the first and second receivers(since they are of no benefit to the third receiver) whilst a phantommode channel can be exploited at all downstream tones, etc.).

Some brief discussion of what is meant by a common indirect channel maybe useful. As per embodiments described below, this can include anunterminated (single ended) phantom channel formed across centre tapvoltages of two different twisted metallic pairs. Since this channel isunterminated (since it is not possible to simultaneously measure andco-process the centre tap voltages of the respective TMP's at thecustomer premises ends receivers in general (since in general these willbe in quite distinct geographical locations)) the “common” channel is infact only common for part of the channel since it terminates atdifferent locations for the different receivers; nonetheless it iscommon in the sense that at least one end of the channel is commonbetween different receivers and the same (combined) signal is propagatedonto that common part of the channel. Moreover, in general, where itstates above (and elsewhere in the present description) that a channelis between a first channel and a second channel, what is actually meantof course is the channel between the transmission from the transmitteronto the transmitter end of the first channel and the reception at thereceiver at the receiver end of the second channel, and similarly wherereference is made to a channel between a first channel and a receiverwhat is actually meant is the channel from the transmitter via thetransmitter end of the first channel to the receiver, etc. In someembodiments, the common indirect channel may be a (possibly alsounterminated/single-ended) Twisted Metallic Pair (TMP).Single-ended/unterminated TMP's can arise where a spare TMP extends to acustomer premises as part of a pair of TMP's even though only one ofthese pairs is actually terminated (and therefore connected at both ofits ends to telephony equipment).

The method can of course be extended to more than two receivers wherethere is a common, indirect channel which cross-couples onto all of therespective lines. Additionally, the method can be used such that sayreceivers one and two benefit simultaneously from a common transmissionover a first indirect channel and receivers two and three benefitsimultaneously from a transmission over a second indirect channel, suchthat the second receiver benefits simultaneously from two separateindirect channels, etc. Other manners of using embodiments of this fifthaspect of the present invention will occur to persons skilled in the artbased on the above examples.

Further aspects of the present invention relate to processorimplementable instructions for causing a processor to carry out themethod of the first aspect of the present invention and/or for causing aprocessor to operate as a phantom channel selector device in accordancewith the fourth aspect of the present invention; aspects of theinvention also relate to carrier media, preferably non-transient,tangible media such as volatile or non-volatile solid state storagemedia (e.g. USB thumb drives etc.), magnetic storage media such as ahard drive, or optical storage media such as a CD or DVD, etc.; carryingsuch processor implementable instructions as mentioned above.

Aspects of the present invention additionally relate to a suitableapparatus for performing the method according to the first aspect of thepresent invention, in particular to a transmitter device fortransmitting data from the transmitter device to a first and a secondreceiver device, the receiver devices being connected, in use, to thetransmitter device via a first and a second pair of wires respectively,each receiver device being operable to receive signals detected as achange over time in the potential difference across the local ends ofeach respective pair of wires extending between the respective receiverdevice and the transmitter device, the transmitter device being operableto transmit signals onto the wires extending between the transmitterdevice and the receiver devices in order to transmit signals via adirect differential mode to each respective receiver via its respectivepair of wires, and is additionally operable to transmit signals to bothreceivers via a single common indirect channel, the transmittercomprising: a channel estimator for estimating the extent of couplingbetween the common indirect channel and each of the receiver devicesbased on readings received by the transmitter device from the receiverdevices; and a processor for determining a plurality of weighting valuesin dependence upon the estimated extents of the couplings; thetransmitter being operable to transmit a first signal via the directdifferential mode over the first pair, to transmit a second signal viathe direct differential mode over the second pair and to transmit acombined signal onto the indirect channel, wherein the combined signalcomprises a weighted sum of the first and second signals, the weightingbeing done in accordance with the determined weighting values; andwherein the transmitter further comprises a precoder for precoding thefirst, second and combined signals to pre-compensate them for theexpected effects of cross-talk from the other ones of these signals,wherein the pre-coding of each signal, including the first and thesecond signals, is performed in dependence upon the determined weightingvalues.

According to a second aspect of the present invention, there is provideda method of transmitting data from a transmitter device to a pluralityof receiver devices, each of which is connected to the transmitterdevice via at least one respective pair of wires, each receiver devicebeing operable to receive signals detected as a change over time in thepotential difference across the local ends of each respective pair ofwires extending between the receiver and the transmitter device, thetransmitter device being operable to transmit signals onto the wiresextending between the transmitter device and the plurality of receiverdevices in a plurality of different modes, over a plurality of differentchannels, the different modes including phantom and differential modesand the different channels including a first set of phantom channels,the method comprising selecting a second set of phantom channels fromthe first set, the second set being a proper subset of the first setcomprising one or some of the phantom channels of the first set (but notall the phantom channels of the first set), the selection being made independence upon the cross-talk coupling between the phantom channels ofthe first set and the reception of signals at each of the receiversdetected as a change over time in the potential difference across thelocal ends of the (or each) respective pair of wires extending betweenthe respective receiver and the transmitter device, connecting theselected phantom channels to the transmitter and transmitting signalsfrom the transmitter onto the (thus selected and connected) phantomchannels of the second set of phantom channels.

It will be apparent that this second aspect enables some lines to havetheir signals strengthened such that improvements can result in theirdata connection (e.g. data rate can be improved, errors can be reduced,latency may be reduced in some circumstances, etc.) and moreover thatsuch improvement can be optimised for a specific desired line or linesto be optimised. It will be apparent to the skilled reader that where itsays “reception” of the signals at each of the receivers detected as achange over time in the potential difference across the local ends ofthe (or each) respective pair of wires extending between the respectivereceiver and the “transmitter” it is clearly conveying the idea that thesignals are received at each receiver in the normal differential mode.Although the second aspect of the invention does not exclude thepossibility that the determination of which phantom channels to employis based upon other more complex considerations (in addition to thesimpler consideration of the crosstalk coupling strengths between thevarious possible phantom channels and the various differential modechannels as detected at the receivers), by basing the analysis at leastupon this latter type of coupling, it is possible for conventionalreceivers, which are only capable of receiving signals via thedifferential mode in respect of a single twisted metallic pair, to beused in the first aspect of the invention. This is important because itmeans that all of the complex functionality for implementing certainpreferred embodiments of the invention can reside in the access network(e.g. at an Access Network Node (ANN) or Digital Subscriber Line AccessMultiplexor (DSLAM), etc.) rather than requiring any special CustomerPremises Equipment (CPE), in certain preferred embodiments of theinvention.

In certain preferred simple embodiments, a special training procedure isused in which signals are transmitted into only a single phantom channel(at any one time) for a given set of receivers (the given set ofreceivers being typically chosen based on some assessment of theirlikelihood to crosstalk interfere with one another at frequencies ofinterest for DSL (including G.FAST) communications with one another—i.e.frequencies which the transmitter and receiver are capable of usingsuccessfully and which the transmitter and receiver (or at least one orsome of the given set of receivers) are permitted to use under localregulations). Each receiver can then measure properties of the receivedtraining signals and feed these back to the transmitter in the normalmanner to thus obtain information about the crosstalk coupling betweenthe single used phantom channel on which the training signals weretransmitted and each of the normal differential mode channels asdetected at each respective receiver. By repeating this trainingprocedure multiple times using different single phantom mode channels itis possible to obtain comprehensive information about the crosstalkcoupling between each such phantom mode channel and each directdifferential mode channel terminating at the receivers of the given setof receivers. This information can then be used to assist in theappropriate selection of which phantom mode channels to use during“showtime” operation of the transmitter and receivers during normal DSLcommunications.

Throughout this specification reference will be made to modes ofcommunication. In this specification the term “mode” is used to indicatethe nature of the manner in which signals are transmitted betweentransmitter and receiver. In particular, as will be appreciated bypersons skilled in the art, there are three principal such modes ofcommunication: differential mode, phantom mode and common mode. In allthree of these modes the signal is transmitted (excited) and received(observed) as the (changing) potential difference (voltage differential)between two voltages (or equivalently between one “live” voltage and one“reference” voltage). In the differential mode the signal istransmitted/observed as the difference in potential between two wires(typically between two wires of a twisted metallic pair). In the phantommode at least one of the voltages is the average voltage of a pair ofwires (note that such average can vary without impacting on a signalcarried in the differential mode across that same pair of wires—in thissense the phantom mode can be orthogonal to signals carried in thedifferential mode if carefully chosen); the term pure phantom mode maybe used to specify that both voltages being compared with each other areaverage voltages, each average voltage being the average or commonvoltage of at least one pair of wires. Second and higher order phantommodes can also be obtained by using the average voltage of two or moreaverage voltages as one of the voltages to be compared, etc. Finally,the common mode refers to the case where one of the voltages beingcompared is the “Earth” or ground reference voltage (or somethingsubstantially similar for telecommunications purposes). Naturally, it ispossible for various mixed modes to also be used for carryingsignals—e.g. one reference voltage could be a common ground and theother could be the average between the voltages of two wires in atwisted metallic pair (to generate a mixed mode of phantom and commonmodes)—however, in general, reference to a differential mode in thisspecification is used to refer to a pure differential mode—i.e. it doesnot include any phantom or common mode component so a mode comprising acomparison between the voltage on a single wire and the average voltagebetween the voltages of two other wires may be referred to as an impurephantom mode rather than a mixed phantom and differential mode, etc.

Preferred embodiments of the present invention are primarily concernedwith the intelligent usage of pure phantom modes, and so in generalreference to a phantom mode will mean such a pure phantom mode be itfirst or second or higher order, etc. unless explicitly specifiedotherwise.

Reference is also made throughout this specification to direct andindirect coupling and direct and indirect channels.

A direct channel is one in which the same physical medium and the samemode of transmission is used for both the transmission of the signal andfor the reception of the signal. Thus a normal differential modetransmission across a single twisted metallic pair from transmitter toreceiver would constitute a direct (differential mode) channel betweenthe transmitter and the receiver.

By contrast, a channel in which the transmitter transmitted a signalonto a second twisted metallic pair in differential mode but wasreceived by a receiver from a first twisted metallic pair indifferential mode (the signal having “crosstalked” across from thesecond to the first pair) is an example of an indirect channel, as is acase in which a signal is transmitted by a transmitter in a phantom modeacross the averages of the voltages of the wires in each of a first andsecond TMP and received (having “crosstalked/mode converted”) by areceiver connected to just the first TMP in differential mode. Manyembodiments described below use such phantom channels which mayhereinafter be referred to as single-ended (or unterminated) phantomchannels (since they can only be directly excited—or observed—at one endof the channel—namely, at the Access Node (AN) side, the signals carriedover such phantom channels only being received at a CPE device to theextent that they have crosstalked/mode converted onto a differentialmode channel).

Moreover, where there are multiple pairs emanating from a singletransmitter (e.g. an Access Node (AN) or DSLAM, etc.) in such a way thatmultiple direct and indirect channels are formed between the transmitterand multiple receivers, the set of twisted metallic channel pairs andtheir derivative channels (direct and indirect and of various differentmodes) can be considered as forming a “unified” dynamic shared orcomposite channel over which a number of virtual channels may beoverlaid (i.e. the virtual channels are overlaid over the underlyingcommon shared channel). In this context, a virtual channel can beconsidered as an overlay channel by which data can be directed toindividual receivers even though a single common underlying signal istransmitted onto the underlying common channel; this can be achieved forexample by means of a suitable multiple access technique such asFrequency Division Multiple Access (FDMA), Code Division Multiple Access(CDMA), Time Division Multiple Access (TDMA) or simply be using suitableencryption techniques, etc. It is interesting to observe, however, thatthis “common” shared channel is comprised of several differentsub-channels which combine together at each receiver/transmitter device(for example a single direct path channel over a twisted metallic pairdirectly connecting the transmitter to the respective receiver, and oneor more indirect, cross-talk paths (possibly also involving modeconversions) of both the differential and the phantom modes from thetransmitter to the receiver via at least one twisted metallic pair whichis connected between the transmitter and another of the receivers). Forthis reason, the dynamic “unified” shared channel is henceforth termed acomposite channel comprising a composition of single-modedirect/indirect couplings/sub-channels and mixed-mode indirectcouplings/sub-channels.

European patent application No. 14 250 116.2 filed by the presentapplicant on 30, Sep. 2014 (BT ref A32607), the contents of which arehereby incorporated, by way of reference, into the present applicationin their entirety, describes a technique for efficiently exploiting sucha common unified channel using virtual overlay channels. Someembodiments of the present invention combine the teachings of thepresent invention application with the teachings of the earlierapplication. In particular, the techniques of the second aspect of thepresent invention are utilised to determine which phantom mode channelsto use and then these are used together with other channels to form acommon unified channel (including the selected phantom modes) over whicha single common signal is transmitted with a suitable multiple accesstechnique being used to provide overlaid virtual channels.

This composite channel (generally) consists of at least two modes:differential and phantom modes. In special scenarios, the common modecan be harnessed and treated in a similar fashion to form additionalsub-channels. In the differential mode, the twisted pairs are made ofdifferential electrical circuits to enable the direct physical linkbetween a transmitter (DPU/DSLAM) and a receiver modem. The co-existenceof multiple twisted pairs in the binder ignites mutual coupling whichresults in immediate and continuous energy dissipation from one pairinto others.

Phantom channels can be constructed from different combinations oftwisted pairs. For instance, a first and a second TMP can togethergenerate a single unique phantom channel which has a similar behaviourto that of each directly coupled differential mode channel formed acrosseach pair in terms of channel directivity. However, phantom modes, asmentioned earlier, are due to the variation of the average voltages ofthe pairs. For more than two coupled pairs, the pairs may couple to eachother in the phantom mode in various orthogonal and non-orthogonalmanners, e.g. 2 distinct (but non-orthogonal) phantom mode channels maybe exploited which share one common pair. Preferred embodiments of theinvention select and construct only orthogonal phantom channels. Thisminimises complex interference effects between the lines whilst stillproviding significant improvements to the lines being targeted forimprovement.

Embodiments of the present invention are based on modelling (so that itcan also be solved) the problem of phantom selection and connection to atransmitter (which combined process may hereinafter be referred to asphantom construction) as a multi-objective optimisation problem(hereinafter referred to as PC-MOP standing for PhantomConstruction—Multi-objective Optimisation Problem). The target of thisoptimisation problem is to obtain an optimal (or at least good) set oforthogonal phantom combinations to maximise the mode conversioncrosstalk onto all pairs. In some preferred embodiments, a Pareto methodis employed to determine the Pareto front which contains the best (or atleast good or close to the best) phantom tree access strategy. Theoptimisation problem can also be biased or weighted to benefit aspecific pair, e.g. worst pairs. In DSL environments, the Pareto frontcan be calculated only once (or at least relatively infrequently) sincethe channel behaviour is considered stationery (or almost stationary).Once the phantom channels have been selected, an analysis is preferablyperformed to determine an exploitation strategy in time, frequency andspace which achieves a certain predetermined objective which, in apreferred embodiment, may include (or consist of) maintaining fairnessconstraints between active users. This approach is advantageous becauseit gives the network operator a degree of flexibility over how toimprove the performance of certain lines (e.g. to improve linesoperating relatively poorly with high errors or high latency or low datarates, etc.).

Single mode crosstalk exploitation (e.g. from TMP 2 differential mode(at transmitter) to TMP 1 differential mode at receiver 1) is lesscomplex than exploiting a phantom mode to differential mode indirectchannel because the single mode crosstalk channels do not need to beconstructed in the way that phantom mode channels must be. Thefundamental issue with single mode crosstalk channels (e.g. from TMP2 toTMP1) is that once a differential mode crosstalk channel is occupied fordata transmission at a specific spectrum, e.g. vectored spectrum, theuser associated with the direct path of that crosstalk channel (e.g.user 2 at the receiver end of TMP 2) becomes inactive, meaning that thevectored spectrum is neglected (even though it might in fact be indemand). Therefore crosstalk channel allocation can be carried out in atime/frequency division multiple access (F/TDMA) fashion when lines arenot in use as in WO2013026479. In certain preferred embodiments of thepresent invention however, the crosstalk transmission knowledge iscapable of being exploited in multiple different ways, depending uponcircumstances and the desired outcome, using techniques such as theprior art techniques known from WO0213026479 in addition to thetechniques taught in the present specification. For example crosstalkchannels can be exploited at some frequencies using a TDMA approachwhilst at other frequencies a Code Division Multiple Access (CDMA)technique could be employed instead. Moreover, the frequencies at whichsuch different techniques are employed can also be changed over time tosuit differing requirements, etc. This provides great flexibility to thesystem and gives the ability to network operators to dynamically adjustthe properties of connections to respond to changes in demand orexternal noise environments etc.

In addition, some preferred embodiments of the invention employ a methodto share a crosstalk channel spatially to benefit multiple (or all)active lines (crosstalk coupled to each other) at a given frequency,simultaneously. This may be done by employing a common optimisationframework for both phantom channels and crosstalk channels except thatthe single-ended phantom channels are allowed to be exploited over anyfrequency without any restriction while crosstalk channels are onlyexploited in the diversity region of the channel above a criticalfrequency (at which it becomes more efficient to use methods such asthose described in EP 14 250 116.2 referred to above by which theconnections between transmitter and receivers are treated as a singlecommon unified channel) except for unused/inactive lines (which areexploited without restriction in the same way as phantom mode channels).Therefore, some embodiments of the invention provide a completeutilisation framework for indirect channels over distinct differentspectrum regions, i.e. vectored, crosstalk and phantom modetransmissions, to enable simultaneous dynamic access. (Also note thatdirect paths may also be optimised in some preferred embodiments of thepresent invention.) This approach again provides great flexibility ofthe system to the network operator to adjust the operation of the linesto account for changes in demand or changes in the noise environmentwithin which the system is operating, etc.

An example preferred method utilising the above ideas is therefore amethod in accordance with the first aspect of the present invention inwhich an indirect channel which is formed from a direct differentialmode of transmission over a pair of wires connected between thetransmitter device and a third receiver device is used to transmit acombined signal comprising a weighted sum of the first and secondsignals at a tone which is not used for transmissions between thetransmitter and the third receiver but is used for transmissions betweenthe transmitter and the first and second receivers.

Further aspects of the invention relate to a transmitter for carryingout the method of the second aspect of the invention. In particular, athird aspect of the present invention provides a transmitter fortransmitting data to a plurality of receiver devices, each of which isconnected to the transmitter device via at least one respective pair ofwires, each receiver device being operable to receive signals detectedas a change over time in the potential difference across the local endsof each respective pair of wires extending between the receiver and thetransmitter device, the transmitter device being operable to transmitsignals onto the wires extending between the transceiver device and theplurality of receiver devices in a plurality of different modes, over aplurality of different channels, the different modes including phantomand differential modes and the different channels including a first setof phantom channels, the transmitter comprising a phantom channelselector for selecting a second set of phantom channels from the firstset, the second set being a proper subset of the first set comprisingone or some of the phantom channels of the first set (but not all thephantom channels of the first set), the selection being made independence upon the cross-talk coupling between the phantom channels ofthe first set and the reception of signals at each of the receiversdetected as a change over time in the potential difference across thelocal ends of the respective pair of wires extending between therespective receiver and the transmitter device; and a connector forconnecting the selected phantom channels to the transmitter such thatthe transmitter is able to transmit signals from the transmitter ontothe (thus selected and connected) phantom channels of the second set ofphantom channels.

It should be noted that described embodiments are couched in terms ofthe downstream direction of data only (i.e. from an Access Node/DSLAM toCustomer Premises Equipment (CPE) devices)—e.g. by referring to atransmitter rather than a transceiver, etc. However, in a practicalimplementation the “transmitter” of the second aspect of the presentinvention also, naturally, functions as a receiver for upstreamtransmissions from the various CPE devices (which are also therefore inpractice operating as transceivers rather than just receivers). However,embodiments of the invention described herein may operate in an entirelyconventional manner in the upstream direction and not exploit phantomchannels in the transmission or reception of upstream signals such thatdiscussion of the upstream aspects would be superfluous and not greatlyassist the reader in understanding the operation of embodiments of thepresent invention.

A fourth aspect of the present invention relates to a phantom channelconnector for connecting a transmitter device to a selected set ofphantom channels carried over a plurality of pairs of wires extendingbetween the transmitter and a plurality of receiver devices, the phantomchannel connector comprising: a phantom channel selection signalreceiver for receiving a phantom channel selection signal specifying aset of one or more (selected) phantom channels, the set of selectedphantom channels comprising a subset of the total number of possiblephantom channels to which the connector is operable to connect to thetransmitter, a set of one or more pairs of input terminals, each pair ofinput terminals being operable to receive a transmission signal fortransmission over an associated selected phantom channel; a switcharrangement; and a plurality of phantom mode driving couplers forelectrically coupling a voltage signal output from the switchingarrangement to a pair of wires in a manner suitable for driving acomponent voltage of a phantom mode signal over the pair of wires;wherein the switching arrangement is operable to selectively couple theor each of one or more of the input terminals to any one of (at least aplurality of) the output terminals in dependence upon the receivedphantom channel selection signal such that, in use, a transmissionsignal applied to a pair of input terminals is capable of beingtransmitted over a selected phantom channel in dependence upon thereceived phantom channel selection signal.

The term “component voltage of a phantom mode signal” is intended torefer to a single one of the two voltage points required to establish apotential difference, the variation of which over time can give rise toa voltage transmission signal. In a simple case of a first order phantomsignal such a component voltage may be the average of the voltagebetween the two wires within a single twisted metallic pair of wires;however, for a second order phantom signal a component voltage would bethe average of the average voltages of the two wires within two distincttwo wire pairs, etc. Preferably, therefore, the driving couplerscomprise centre tap connections to an inductor or transformer connectedto a pair of wires at the transmitter end of the wires. This provides asimple and robust manner of accessing the first order phantom channelsof the attached twisted metallic pairs. In other words, the phantom modedriving couplers are operable to electrically couple a voltage signaloutput from the switching arrangement to a plurality of pairs of wiresin a manner suitable for driving a phantom mode signal over the pairs ofwires.

The phantom channel connector of the fourth aspect of the presentinvention not only permits a selected phantom channel or channels to beexploited for the benefit a particular CPE device or devices,additionally, it allows different such phantom channels to be selectedquickly and easily based on a received phantom channel selection signal.This not only assists in selecting an appropriate phantom channel to usein any particular given circumstances, but also enables phantom channelsto be selected individually for training purposes as well as in sets foruse once training has completed, etc.

A fifth aspect of the present invention relates to a phantom channelselector device, which may either be a part of a transmitter device(such as, for example, the transmitter device according to the thirdaspect of the present invention) or else be communicably attachedthereto, and which is operable to select, from a first set of possiblephantom channels, a second set of selected phantom channels, the secondset being a proper subset of the first set, the second set comprisingone or more phantom channels carried over a plurality of pairs of wires,each of which pair of wires extends between the transmitter device andone of a plurality of receiver devices, on to each of which selectedphantom channel the transmitter device is to transmit a transmissionsignal, the phantom channel selector device comprising: a coupling datareceiver for receiving receiver signal reception data and/or crosschannel coupling data; a selection interface for communicating a phantomchannel selection signal or message to a phantom channel connector (suchas, for example the phantom channel connector according to the thirdaspect of the present invention), and a processor arranged to generate aphantom channel selection for communication to the phantom channelconnector within the phantom channel selection signal or message independence upon the received signal reception data and/or cross channelcoupling data.

Preferably, the processor is operable to generate the phantom channelselection by solving a pareto-based multi-objective optimisationproblem.

Thus in a preferred embodiment, there is provided a phantom channelselector device, forming part of a transmitter device, the phantomchannel selector device being operable to select a plurality of phantomchannels carried over a plurality of pairs of wires extending betweenthe transmitter and a plurality of receiver devices on to which totransmit a transmission signal or signals, the phantom channel selectordevice comprising: a coupling data receiver for receiving receiversignal reception data and/or cross channel coupling data; a selectioninterface for communicating a phantom channel selection signal and/ormessage to a phantom channel connector (such as the phantom channelconnector according to the third aspect of the present invention), and aprocessor arranged to generate a phantom channel selection forcommunication to the phantom channel connector within the phantomchannel selection signal and/or message in dependence upon the receivedsignal reception data and/or cross channel coupling data and the phantomchannel selection signal and/or message.

Preferably, the phantom channel selector device further comprises amulti-objective problem processing unit for performing a determinationof which phantom channels to select as the solution of a multi-objectiveproblem in which a solution is sought to simultaneously benefit two ormore of the receivers.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be better understood,embodiments thereof will now be described with reference to theaccompanying drawings in which:

FIG. 1 is a schematic illustration of an example broadband connectiondeployment showing a Distribution Point Unit (DPU) and two customerpremises having associated Customer Premises Equipment (CPE) modemsconnected to the DPU via respective Twisted Metallic Pair (TMP)connections;

FIG. 2 is a schematic block diagram illustrating the principalcomponents in a modem to modem connection operating in accordance with afirst embodiment of the present invention;

FIG. 3 is a schematic block diagram of the Multiple Phantom AccessDevice (MPAD) of FIG. 2, illustrating the device in greater detail;

FIG. 4 is a schematic block diagram similar to FIG. 3, illustrating analternative Multiple Phantom Access Device (MPAD) which is suitable foruse with four rather than three wire pairs; and

FIG. 5 is a graph illustrating an example pareto front for a simple caseconcerning selecting optimal phantom channels for use in assisting twodifferent wire pairs/receivers.

SPECIFIC DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates in overview an example broadband deployment in whichembodiments of the present invention could be employed. As shown in FIG.1, the example deployment comprises a Distribution Point Unit (DPU) 10which is connected to three user premises 31, 32, 33 (which in thisexample are flats within a single house 30) via respective TwistedMetallic Pair (TMP) connections 21, 22, 23 which connect between anAccess Node (AN) 16 (e.g. a Digital Subscriber Line Access Multiplexor(DSLAM)) within the DPU 10 and respective Customer Premises Equipment(CPE) modems 51, 52 via respective network termination points 41, 42within the respective customer premises 31, 32. The DPU 10 additionallyincludes an Optical Network Termination (ONT) device 14 which provides abackhaul connection from the DPU 10 to a local exchange building via anoptical fibre connection such as a Passive Optic-fibre Network (PON) anda controller 12 which coordinates communications between the AN 16 andthe ONT 14 and which may perform some management functions such ascommunicating with a remote Persistent Management Agent (PMA).

As will be apparent to a person skilled in the art, the illustrateddeployment involving an optical fibre backhaul connection from adistribution point and a twisted metallic pair connection from thedistribution point to the “customers” premises is exactly the sort ofdeployment for which the G.FAST standard is intended to be applicable.In such a situation, the TMP connections may be as short as a fewhundred metres or less, for example possibly a few tens of metres onlyand because of this it is possible to use very high frequency signals(e.g. up to a few hundred Megahertz) to communicate over the short TMP'sbecause the attenuation of high frequency signals is insufficient toprevent them from carrying useful information because of the shortnessof the lines. However, at such high frequencies crosstalk becomes asignificant issue. This is clearly especially going to be the case wherethe cross-talking lines travel alongside each other for part of theirextent (as in the situation illustrated in FIG. 1); however, cross-talkis still an issue at high frequencies (e.g. over 80 MHz) even where thelines only lie close to one another for a very small portion of theirtotal extent (e.g. just when exiting the DPU 10). G.FAST currentlyproposes simply using vectoring techniques at all frequencies wherethere are cross-talking lines in order to mitigate against thecross-talk effects.

In addition, in this scenario, by accessing at the DPU 10 (in particularat the Access Node (AN) 16) phantom channels, it is possible to exploitsignals transmitted onto phantom channels which will “crosstalk” ontothe conventional differential mode channels associated with each of theend user receivers (the termination point and CPE modem combinations41/51, 42/52, 43/53) and change the signals received (compared to aconventional case where the phantom channels are not exploited in thisway). Since there are three TMP connections 21-23, there are 3 possible(first order, pure) phantom channels which could be exploited in thisway, formed by using the differential voltage signal between: theaverage voltage of TMP 21 and that of TMP 22; the average voltage of TMP21 and that of TMP 23; and the average of TMP 22 and that of TMP 23.However, since there is no possible set of two of these possible (firstorder, pure) phantom channels which does not include at least one commonTMP, only one of these can be used at the same time without havingnon-orthogonal (and hence complexly interfering) phantom channels beingused simultaneously. Thus the present embodiment includes a PhantomChannel—Multiple Optimisation Problem device (PC-MOP) which, as isexplained in greater detail below, acts to choose a single one out ofthe three possible phantom channels to use—the selection being performedsuch as to try to achieve a particular set of two (or more) objectives(e.g. to try to obtain the maximum benefit for two of the threereceivers).

Referring now to FIG. 2, there is shown a schematic illustration of theprincipal components within the AN 16 and CPE modems 51, 52, 53 allowingthe indirect phantom channels to be utilised according to a first simpleembodiment chosen to illustrate the basic principles of the approach.

As shown, the AN 16 according to the embodiment illustrated in FIG. 2comprises first, second and third Data Source, Data Encoder and Serialto Parallel converter (DSDESP) modules 1611, 1612 and 1613. These areessentially conventional functions within a DSL modem and will not befurther described here except to point out that each one's output is aset of data values d₁-d_(M) each of which can be mapped to both a set ofone or more bits and to a point within a modulation signal constellationassociated with a respective tone on which the data value is to betransmitted. For example if a tone t₁ is determined to be able to carry3 bits of data a corresponding data value will be set to one of 2³=8different values (e.g. to a decimal number between 0 and 7) each ofwhich corresponds to a different constellation point within anassociated signal constellation having 8 different constellation points.The data values for a single symbol can be thought of as forming avector of data values (one for each data-carrying tone) and togethercarry the user data to be transmitted to the end user associated with arespective end user modem 51, 52, 53 together with any overhead data(e.g. Forward Error Correction data etc.).

(N.B. It is worth noting that the assessment of the number of bits whichany particular tone for any particular receiver may carry (per symbol)should be done with the benefit of the usage of any assisting phantommode channels (as discussed below) and the benefit of vectoring takeninto account. Thus it should be borne in mind that the presentdiscussion relates to “Showtime” operation of the system once alltraining procedures have been completed. In overview the traininginvolves firstly determining which phantom channel (or channels inembodiments in which more than one phantom channel can be exploited atthe same time—e.g. for embodiments in which more than 3 lines areconnected to a common AN and are sufficiently closely cross-talk coupledto make exploitation of the phantoms worthwhile) to use and then settingparameters for its usage. Having determined how to best exploit thephantom channels, then the training continues by performing vectoringtraining to determine the vectoring parameters to use and thendetermining the number of bits which can be used with both assistancefrom the phantom channel(s) and from vectoring.)

The data values leaving each DSDESP module 1611, 1612, 1613 are thenpassed (in an appropriate order) to respective Multiple bit levelQuadrature Amplitude Modulation (M-QAM) modulators 1621, 1622, 1623which convert each input data value to a respective complex number x₁ ¹to x_(M) ¹, x₁ ² to x_(M) ² and x₁ ³ to x_(M) ³ each of which representsa complex point within a complex number constellation diagram. Forexample a data value d₁ ¹=7 (=111 in binary) might be mapped by theM-QAM modulator 1621 to the complex number 1-i for tone 1 where tone 1has been determined (by say modem 51) to be able to carry 3 bits of dataeach.

Each of these complex numbers x₁ ¹ to x_(M) ¹, x₁ ² to x_(M) ² and x₁ ³to x_(M) ³ is then entered into a vectoring precoder module 1630 (whichin the present embodiment is a single common vectoring precoder module1630) which performs a largely conventional vectoring operation in orderto precode the transmissions to be sent using a combination ofpredetermined vectoring coefficients and information about the signalsto be transmitted onto the other lines within the relevant vector groupin a manner, which is well known to those skilled in the art, tocompensate for the expected effects of cross-talk from the other linesin the vector group. The vectoring precoder module differs from aconventional vectoring precoder module in that it is operable toadditionally precode the transmissions in such a way as to cause them tobe pre-compensated for the expected crosstalk effects produced not onlyby the neighbouring lines operating in a direct differential mode (asper standard vectoring), but also for the effects of crosstalk comingfrom any signals being transmitted onto one or more phantom channels (orother channels which are not direct differential mode channels). Inorder to do this (as will become apparent form the detailed descriptionbelow) it is necessary for the vectoring precoder module 1630 to receiveinformation about channel estimations of the respective phantomchannel(s) (or other channels which are not direct differential modechannels) and also information about any weighting values used tocombine signals to be transmitted over the phantom channel(s) (or otherchannels which are not direct differential mode channels). The outputfrom the vectoring precoder module 1630 is thus a set of furthermodified complex numbers i: {circumflex over (x)}₁ ¹ to {circumflex over(x)}_(M) ¹, {circumflex over (x)}₁ ² to {circumflex over (x)}_(M) ² and{circumflex over (x)}₁ ³ to {circumflex over (x)}_(M) ³.

The ability of the vectoring precoder module 1630 to receive theweighting values and channel estimation values which it needs to performits precoding functions is illustrated in FIG. 2 by the line between thePC-MOP & MICOP & MRC & Management entity module 1690 (which performsgeneral management functions in addition to its specific functionsdescribed in greater detail below and for brevity may hereinafter bereferred to either as the “management entity” or the “PC-MOP module”)and the vectoring precoder module 1630. In the present embodiment, thePC-MOP module calculates appropriate values for the channel estimationsand the weighting values required by the vectoring precoder module andthe MICOP & MRC precoder module 1640. In order to do this, it needs datareported back to it from the end user modems. The processes andprocedures for achieving this are largely conventional and well known topersons skilled in the art and so they are not discussed in great detailherein except to note that it relies on a backward path from thereceivers 51,52,53 to the transmitter 16. This is achieved in practice,of course, in that the receivers 51,52,53 are in practice transceiverscapable of receiving and transmitting signals over the TMP's 51,52,53 asis the transmitter 16—the receiver parts of the transmitter 16 and thetransmitter parts of the receivers 51,52,53 have simply been omittedfrom the drawings to avoid unnecessary complication of the figuresbecause these parts are entirely conventional and not directly pertinentto the present invention. Moreover, each of the receivers additionallycontains a management entity responsible for performing variousprocessing and communication functions. Any of a number of suitabletechniques can be employed for obtaining data useful in generatingchannel estimations. For example, known training signals can betransmitted onto selected channels by the transmitter 16 during aspecial training procedure and the results of detecting these by thereceivers 51,52,53 can be sent back to the transmitter in a conventionalmanner. Additionally, special synchronisation symbols can betransmitted, interspersed with symbols carrying user data, atpredetermined “locations” within a “frame” comprising multiple symbols(e.g. at the beginning of each new frame) and the results of attemptingto detect these synchronisation symbols can also be sent back to thetransmitter to generate channel estimation values. As is known topersons skilled in the art, different synchronisation signals/symbolscan be sent over different channels simultaneously and/or at differenttimes etc. so that different channel estimations (including importantlyindirect channels and indirect channels can be targeted and evaluated,etc.

As will be appreciated by those skilled in the art, the output of thevectoring precoder module 1630 is a set of modified (or predistorted)complex numbers {hacek over (x)}₁ ¹ to {hacek over (x)}_(M) ¹, {hacekover (x)}₁ ² to {hacek over (x)}_(M) ² and {hacek over (x)}₁ ³ to {hacekover (x)}_(M) ³ as mentioned above. These complex numbers are thenpassed to a Mixed-Integer Convex Optimisation Problem and Maximal RatioCombiner (MICOP and MRC) precoder module 1640 (hereinafter referred toas the MICOP and MRC precoder module 1640) which, in the presentembodiment, uses weighting values together with channel estimationvalues provided to it by the PC-MOP module 1690 to calculate, from themodified complex numbers received from the vectoring pre-coder module1640 (and the weighting values and channel estimation values from thePC-MOP module 1690), further modified (or further pre-distorted) valuesfor the complex numbers to be passed to the IFFTs 1651-1652. Note thatin addition to further modifying the received numbers {hacek over (x)}₁¹ to {hacek over (x)}_(M) ¹, {hacek over (x)}₁ ² to {hacek over (x)}_(M)² and {hacek over (x)}₁ ³ to {hacek over (x)}_(M) ³ to generatecorresponding further modified complex numbers {umlaut over (x)}₁ ¹ to{umlaut over (x)}_(M) ¹, {umlaut over (x)}₁ ² to {umlaut over (x)}_(M) ²and {umlaut over (x)}₁ ³ to {umlaut over (x)}_(M) ³ which are to form(ultimately) the signals to be used in driving the respective TMPs 21,22, 23 in direct differential mode, the MICOP and MRC precoder module1640 additionally generates a new set of complex numbers {umlaut over(x)}₁ ⁴ to {umlaut over (x)}_(M) ⁴ which are to form (ultimately) thesignals to be used to drive a (single ended) phantom mode channel to beaccessed via the MPAD module described below. The precise way in whichthis is done is described below with reference to appropriate equations.Once these values have been calculated by the MICOP and MRC precoder1640 they are passed to the respective IFFT modules 1651-1654(super-script 1 values going to IFFT 1651, superscript 2 values going toIFFT 1652, etc.) and the next two steps of the processing areconventional and not relevant to the present invention. Thus each set ofgenerated values (e.g. {umlaut over (x)}₁ ¹ to {umlaut over (x)}_(M) ¹is formed by the respective IFFT module into a quadrature time domainsignal in the normal manner in Orthogonal Frequency DivisionMultiplexing (OFDM)/DMT systems). Then the time domain signals areprocessed by a suitable Analogue Front End (AFE) module 1661 to 1664again in any suitable such manner including any normal conventionalmanner. After processing by the AFE module 1650, the resulting analoguesignals are passed to the MPAD module 1670 (note MPAD stands forMultiple Phantom Access device).

The MPAD module is described in greater detail below, but in overview itprovides switchable access to centre taps of any of the TMPs such thatany of the possible phantom channels associated with the connected linescan be driven by the incoming signal arriving from AFE 1664 as well asdirectly passing on the signals from AFE's 1661-1663 directly to TMPs21-23 for driving in the normal direct differential mode.

During transmission over the TMP connections 21, 22, 23 the signals willbe modified in the normal way according to the channel response of thechannel and due to external noise impinging onto the connections. Inparticular there will be cross-talking (and most particularly far-endcross-talking) between the three direct channels (the direct channelsbeing one from the transmitter 16 to the modems 41-43 via the TMPs 21-23and the phantom channel. However, the effect of the precoding is tolargely precompensate for the effects of the cross talk. Additionally,the targeted receivers additionally benefit from increased SNR of thereceived signal destined for them arriving via cross talk from thephantom channel.

After passing over the TMP connections 21, 22, 23 the signals arereceived by the modems 41-43 at a respective Analogue Front End (AFE)module 5150, 5250, 5350 which performs the usual analogue front endprocessing. The thus processed signals are then each passed to arespective Fast Fourier Transform (FFT) module 5140, 5240, 5340 whichperforms the usual conversion of the received signal from the timedomain to the frequency domain. The signals leaving the FFT modules5140, 5240, 5340, y₁ ¹ to y_(M) ¹, y₁ ² to y_(M) ² and y₁ ³ to y_(M) ³are then each passed, in the present embodiment, to a respectiveFrequency domain EQualiser (FEQ) module 5130, 5230, 5330. The operationof such frequency domain equaliser modules is well-known in the art andwill not therefore be further described herein. It should be notedhowever, that any type of equalisation could be performed here, such asusing a simple time-domain linear equalizer, a decision feedbackequaliser, etc. For further information on equalisation in OFDM systems,the reader is referred to: “Zero-Forcing Frequency-Domain Equalizationfor Generalized DMT Transceivers with Insufficient Guard Interval,” byTanja Karp, Steffen Trautmann, Norbert J. Fliege, EURASIP Journal onApplied Signal Processing 2004:10, 1446-1459.

Once the received signal has passed through the AFE, FFT and FEQmodules, the resulting signals, {umlaut over (x)}₁ ¹ to {umlaut over(x)}_(M) ¹, {umlaut over (x)}₁ ² to {umlaut over (x)}_(M) ² and {umlautover (x)}₁ ³ to {umlaut over (x)}_(M) ³ should be similar to the complexnumbers x₁ ¹ to x_(M) ¹, x₁ ² to x_(M) ² and x₁ ³ to x_(M) ³ originallyoutput by the M-QAM modulators 1621-1623 except that there will be somedegree of error resulting from imperfect equalisation of the channel andthe effect of external noise impinging onto the lines duringtransmission of the signals between the AN and the modems 41-43. Thiserror will in general differ from one receiving modem to the next. Thiscan be expressed mathematically as {umlaut over (x)}_(m) ¹=x_(m) ¹+e_(m)¹ etc. Provided the error however is sufficiently small the signalshould be recoverable in the normal way after processing by the M-QAMdemodulator modules 5120-5320 where a corresponding constellation pointis selected for each value {umlaut over (x)}_(m) ^(i) in dependence onits value (e.g. by selecting the constellation point closest to thepoint represented by the value {umlaut over (x)}_(m) ^(i) unless trelliscoding is being used, etc.). The resulting data values {umlaut over(d)}₁ ¹ to {umlaut over (d)}_(M) ¹, {umlaut over (d)}₁ ² to {umlaut over(d)}_(M) ² and {umlaut over (d)}₁ ³ to {umlaut over (d)}_(M) ³ shouldmostly (apart from some small number of incorrectly detected valuesresulting from errors) correspond to the data values, {umlaut over (d)}₁¹ to {umlaut over (d)}_(M) ¹, {umlaut over (d)}₁ ² to {umlaut over(d)}_(M) ² and {umlaut over (d)}₁ ³ to {umlaut over (d)}_(M) ³originally entered to the corresponding M QAM modules 1621,1622, 1623respectively within the AN/transmitter 16. These values are then enteredinto a respective decoder (and received data processing) module 5110,5210 and 5230 which reassembles the detected data and performs anynecessary forward error correction etc. and then presents the recovereduser data to whichever service it is addressed to in the normal manner,thus completing the successful transmission of this data.

As mentioned above, following now from the above overview of FIG. 2, amore detailed explanation is provided of the non-conventional elementswithin the embodiment illustrated in FIG. 2 and described briefly above.Thus, the MPAD 1670 is a component which provides access to differentcombinations of phantom channels. MPAD 1670 tries all the possiblecombinations without repetition, e.g. phantom of pair 1 and pair 2 isequivalent to the phantom of pair 2 and pair 1 and so will not berepeated). Herein, MPAD (1670) selects a specific phantom and it allowsthe transmitter 16 and each respective receiver 51, 52, 53 to train upwith each other and obtain the phantom channel as well as the directdifferential mode pairs' channel coefficients at any given specific timeslot. At this stage the receivers 51, 52, 53 report either the overallcombined channel or the phantoms only to the PC-MOP module 1690depending on what signals are transmitted by the transmitter 16 which isdone under the control of the PC-MOP module so that it knows what datais being reported back to it by the receivers. At the same time theInterface 1680 confirms the identification of the selected and currentlyoperational phantom channel to PC-MOP module 1690 (which is alsoselected by the interface 1680 under instruction from the PC-MOP module)so that all channel gains and their identifications are capable of beingascertained by PC-MOP 1690 for subsequently passing to the vectoringprecoder module 1630 and the MICOP & MRC precoder module 1640 for use inperforming their precoding functions. The operation continues until allthe phantom channels' combinations are tested. Once the phantom tree iscompleted, PC-MOP 1690 decides the optimal phantom channels to beexploited to benefit specific pairs, all the pairs or to maximise therate equilibrium of the users. The decision is then forwarded to theMPAD module 1670 via the Interface 1680 to execute the decision andenable the access to the selected optimal phantom channel (or channelsin alternative embodiments where the MPAD connects to more than 3 TMPs).

Once the optimum phantom channel is “constructed” and ready to beaccessed, MICOPMRC module 1640 then decides the optimal strategy to“steer” the constructed phantoms. This is done by selecting appropriateweighting values as described in greater detail below. The steeringobjective can be modified to maximise a specific pair or the rateequilibrium or any other desired objective.

There now follows a mathematical explanation of the functioning of thevarious elements. In some cases the equations deal only with two directdifferential mode signals and one phantom mode signal; however, it willbe apparent to a person skilled in the art how to expand this to covermultiple different direct differential signals and multiple phantomsignals based on the following example expositions. Thus, considering asystem with K twisted pairs, each pair denoted by tp_(i) where i, i∈K isthe pair's index, there are

$M = \left\lfloor \frac{K}{d} \right\rfloor$first order orthogonal phantoms, where d is the required number of pairsto construct a single phantom channel. Similar rule applies for secondorder phantoms and so on until the orthogonal phantom tree is fullyobtained. The total number of the first order orthogonal phantomcandidates can be calculated by

$\begin{pmatrix}K \\d\end{pmatrix} = \frac{K!}{{d!}{\left( {K - d} \right)!}}$and we will consider this as the feasible domain for the PC-MOP problem,denoted by Φ. The standard conventional channel is given as:

$H = \begin{pmatrix}h_{1,1} & h_{1,2} & \ldots & h_{1,K} \\h_{2,1} & h_{2,2} & \ldots & h_{2,K} \\\vdots & \vdots & \ddots & \vdots \\h_{K,1} & h_{K,2} & \ldots & h_{K,K}\end{pmatrix}$where h_(i,j) indicates the channel transfer function for thetransmission by the transmitter onto the j^(th) TMP (or phantom channelwhen extended as described immediately below) to the i^(th) receiver asreceived at the i^(th) receiver over the i^(th) TMP or tp (=twistedpair).

A phantom channel (ϕ_(m), ∀m∈M) is derived from a pair of tp, i.e.{tp_(i), tp_(j)}_(i≠j)

TABLE 1 First order phantom mode candidates {tp_(i), tp_(j)} _(i≠j)h_(k,ϕ) _(m) {1, 2} {1, 3} {1, 4} {1, 5} {2, 3} {2, 4} {2, 5} {3, 4} {3,5} {4, 5} h_(5,ϕ) _(m) 0.5 0.3 0.4 0.45 0.25 0.1 0.3 0.2 0.3 0.2 h_(4,ϕ)_(m) 0.3 0.3 0.2 0.4 0.52 0.6 0.3 0.3 0.4 0.5,

∀i & j∈K when d is 2. Hence the extended channel becomes:

$\left\lbrack H \middle| H_{\Phi} \right\rbrack = {H_{T} = \begin{pmatrix}h_{1,1} & h_{1,2} & \ldots & h_{1,K} & h_{1,\phi_{1}} & \ldots & h_{1,\phi_{M}} \\h_{2,1} & h_{2,2} & \ldots & h_{2,K} & h_{2,\phi_{1}} & \ldots & h_{2,\phi_{M}} \\\vdots & \vdots & \ddots & \vdots & \vdots & \ddots & \vdots \\h_{K,1} & h_{K,2} & \ldots & h_{K,K} & h_{K,\phi_{1}} & \ldots & h_{K,\phi_{M}} \\h_{\phi_{1},1} & h_{\phi_{1},2} & \ldots & h_{\phi_{1},K} & h_{\phi_{1},\phi_{1}} & \ldots & h_{\phi_{1},\phi_{M}} \\\vdots & \vdots & \ddots & \vdots & \vdots & \ddots & \vdots \\h_{\phi_{M},1} & h_{\phi_{M},2} & \ldots & h_{\phi_{M},K} & h_{\phi_{M},\phi_{1}} & \ldots & h_{\phi_{M},\phi_{M}}\end{pmatrix}}$where H_(Φ) is the phantom channel, H is the unextended channel(excluding phantom channels) and H_(T) is the mixed mode channel.Herein, the PC-MOP can be formulated as follows:max H_(Φ,)  (1)subject to:ϕ_(m)∈Φ  (2)

To illustrate the selection strategy of Pareto, we provide the followingexample: Assume a 5 pair cable in which pairs 5 and 4 are performingpoorly in comparison to pairs 1, 2 and 3. Therefore, the phantoms may bederived and steered to maximise the performance of pairs 4 and 5.Maximum number of the first order orthogonal phantoms is

$\left\lfloor \frac{5}{2} \right\rfloor = 2$and the maximum number of combinations is

$\begin{pmatrix}5 \\2\end{pmatrix} = {\frac{5 \times 4 \times 3 \times 2 \times 1}{2 \times 1\left( {3 \times 2 \times 1} \right)} = 10.}$Table 1 shows all the orthogonal phantom candidates and theirmode-conversion crosstalk coefficient with the targeted pairs. To obtainPareto front, we must determine the non-dominant solution, i.e. Paretofront. To examine the dominance of a set, it must contain at least oneelement greater than an element in another set if the objective functionis set to maximisation. In this particular example, {1,2} dominates{1,3}, {1,4}, {2,5} and {3,4}. Similarly, candidates {1,5}, {2,3} and{2,4} dominant {1,3}, {1,4}, {2,5} and {3,4}. Hence {1,2}, {1, 5}, {2,3}and {2, 4} are the non-dominant solution and known as the Pareto front,see the example Figure below.

In a similar way, the objective function can include more pairs tobenefit from the phantoms, also the phantom directivity can be alteredto optimise the direct paths of the phantom mode if they are accessibleat the receiving end, i.e. direct phantom channels. This remains thechoice of the network operator. Since predicting the phantom couplingstrength from first principles is an arduous task, in the presentembodiment, PC-MOP 1690 proceeds by simply initialising all possiblephantom channels randomly in a non-repetitive pattern. Alternatively,however, one could also model the phantoms and predict their performancein advance and select the optimal combination without the randomtraining in alternative embodiments.

Once the phantoms are defined, it is advantageous to try to determinethe optimal strategy to steer and split the indirect channels tomaximise the overall binder capacity whilst fairness constraints betweenthe users are kept satisfied. To achieve this, the indirect(phantom/crosstalk) channel utilisation problem is formulated as aMixed-Integer Convex OPtimisation (MICOP) model in order to enable thePC-MOP 1690 to then derive a solution.

In order to simplify the problem, to illustrate the operation of thePC-MOP 1690, consider a single phantom to be shared among K users totransmit N tones for a period of time T. Power level per tone is denotedby P_(k,t,n) and the channel condition is γ_(k,t,n) which is the ratioof power coupling coefficient to the noise level

$\left( \frac{{h_{k,\phi_{m}}}^{2}}{n_{k,\phi_{m}}} \right)_{t,n}.$The tone allocation factor is ρ_(k,t,n) and finally the optimal capacityof the m^(th) phantom is C_(ϕ) _(m) ,

$\begin{matrix}{{{\max\; C_{\phi_{m}}} = {\sum\limits_{k,t,n}^{\;}{\rho_{k,t,n}{\log_{2}\left( {1 + {p_{k,t,n}\gamma_{k,t,n}}} \right)}}}},} & (3)\end{matrix}$

-   -   subject to:

$\begin{matrix}{{{\sum\limits_{k,n}^{\;}p_{k,t,n}} \leq P_{\phi\; m}},{\forall{t \in T}}} & \left( {4a} \right) \\{{{\sum\limits_{t,n}^{\;}{\rho_{k,t,n}{\log_{2}\left( {1 + {p_{k,t,n}\gamma_{k,n}}} \right)}}} \leq R_{k}},{\forall{k \in K}}} & \left( {4b} \right) \\{{{\sum\limits_{k,t}^{\;}\rho_{k,t,n}} \leq T},{\rho \in \left\{ {0,1} \right\}},{\forall{n \in N}},} & \left( {4c} \right)\end{matrix}$Equation (3) is the objective function in which its limit is subject tothe maximum transmitting power in 4a and the tone sharing criteria in4c.

The optimisation problem in its current form is non-linear with no knownanalytical solution. However, a simple modification has been applied to3.

$\begin{matrix}{{{\max\; C_{\phi_{m}}} = {\sum\limits_{k,t,n}^{\;}{\rho_{k,t,n}{\log_{2}\left( {1 + \frac{s_{k,t,n}\gamma_{k,t,n}}{\rho_{k,t,n}}} \right)}}}},} & (5)\end{matrix}$

-   -   subject to:

$\begin{matrix}{{{\sum\limits_{k,n}^{\;}s_{k,t,n}} \leq P_{\phi\; m}},{\forall{t \in T}}} & \left( {6a} \right) \\{{{\sum\limits_{t,n}^{\;}{\rho_{k,t,n}{\log_{2}\left( {1 + \frac{s_{k,t,n}\gamma_{k,t,n}}{\rho_{k,t,n}}} \right)}}} \leq R_{k}},{\forall{k \in K}}} & \left( {6b} \right) \\{{{\sum\limits_{k}^{\;}\rho_{k,n}} \leq 1},{\forall{n \in M}},{\forall{t \in T}}} & \left( {6c} \right)\end{matrix}$

The modified problem in 5 is concave and hence it is solvable as aconvex problem. This problem as it stands provide the optimal TDMA andFDMA access to the phantoms. The analytical solution proceeds with theLagrangian as follows:

$\begin{matrix}{{\mathcal{L} = {{\sum\limits_{k,t,n}^{\;}{\rho_{k,t,n}{\log_{2}\left( {1 + \frac{s_{k,t,n}\gamma_{k,t,n}}{\rho_{k,t,n}}} \right)}}} - {\sum\limits_{t}^{\;}{\Omega_{t}\left( {{\sum\limits_{k,n}^{\;}s_{k,t,n}} - P_{\phi_{m}}} \right)}} - {\sum\limits_{t,n}^{\;}{\mu_{t,n}\left( {{\sum\limits_{k}^{\;}\rho_{k,t,n}} - 1} \right)}} - {\sum\limits_{k}^{\;}\left\lbrack {{\sum\limits_{t,n}^{\;}{\rho_{k,t,n}{\log_{2}\left( {1 + \frac{s_{k,t,n}\gamma_{k,t,n}}{\rho_{k,t,n}}} \right)}}} - R_{k}} \right\rbrack}}},} & (7)\end{matrix}$

To solve 7 and prove its optimality, Karush Kuhn Tucker (KKT) conditionsmust be satisfied. The conditions are:

-   -   1. Feasibility of the primal constraints as well as the        multipliers, i.e. (Ω & μ).    -   2. The gradient of 7 must become zero with respect to 6a and 4c.

Starting by differentiating 7 with respect to s_(k,n), i.e.

${\frac{\partial\mathcal{L}}{\partial s_{k,n}} = 0},$then rearrange to obtain the optimal power formula:

$\begin{matrix}{{p_{k,t,n} = {{\lambda_{t}\left( {1 - \beta_{k}} \right)} - \frac{1}{\gamma_{k,t,n}}}},} & (8)\end{matrix}$where

$\lambda_{t} = {\frac{1}{\ln\; 2\Omega_{t}}.}$To guarantee feasible 8 and 4a,

$\frac{1}{\gamma_{k,t,n}} \leq \lambda_{t} \leq {\frac{P_{\phi_{m}} + {\sum\limits_{k,n}^{\;}\frac{1}{\gamma_{k,t,n}}}}{\sum\limits_{k,n}^{\;}\left( {1 - \beta_{k}} \right)}.}$

The sharing factor can be used simply to guarantee that a single tonecan only be assigned to a single user, e.g. tone 1 assigned to user 1 isrepresented by ρ_(1,1)=1 and elsewhere ρ_(k≠1,1)=0. Hence constraint 4cis relaxed to:

$\rho_{k,n} = \left\{ {\begin{matrix}1 & {{{if}\mspace{14mu}{the}\mspace{14mu} n^{th}\mspace{14mu}{tone}\mspace{14mu}{is}\mspace{14mu}{assigned}\mspace{14mu}{to}\mspace{14mu}{the}\mspace{14mu} k^{th}\mspace{14mu}{user}},} \\0 & {elsewhere}\end{matrix}.} \right.$

In a similar fashion to 8, we differentiate 7 with respect ρ_(k,n),rearrange and substitute 8 to obtain the following:

$\begin{matrix}{{\mu_{t,n} = {{\log_{2}\left\lbrack {{\lambda_{t}\left( {1 - \beta_{k}} \right)}\gamma_{k,t,n}} \right\rbrack} - {\frac{1}{\ln\; 2}\left\lbrack {1 - \frac{1}{{\lambda_{t}\left( {1 - \beta_{k}} \right)}\gamma_{k,t,n}}} \right\rbrack}}},{\forall{n \in N}}} & (9)\end{matrix}$

The user which maximises 9 for tone n represents the optimal user. Hencek is obtained by:{circumflex over (k)}=arg max μ_(t,n,∀t∈T,∀n∈N)  (10)and therefore, by assessing which k (i.e. which end CPE receiver) toselect for each tone, n, a weighting value, of (in this embodiment) zeroor one, is determined for each line, at each tone, in dependence uponthe measured extent of the couplings between a phantom mode channel,φ_(m) and each differential mode channel k as determined by a receiverreceiving signals in the differential mode (recall that

$\left. {\gamma_{k,t,n} = \left( \frac{{h_{k,\phi_{m}}}^{2}}{n_{k,\phi_{m}}} \right)_{t,n}} \right).$

Similarly to section 1, the method can be applied to the differentiallines except that the k domain is limited to each line itself. Hence,the binder capacity in total, becomes:

$\begin{matrix}{{C_{binder} = {{\sum\limits_{p\; h}^{\;}C_{\phi_{m}}} + {\sum\limits_{dif}^{\;}C_{dif}}}},} & (11)\end{matrix}$

once the phantom sharing and power allocation policies are obtained. Thepower allocation per line needs to be re-configured to ensure that thephantom gain results in (or at least does not exceed) the capacity gain.The optimisation problem is similar to 5 excluding 4c. Line's channelgain in the presence of phantom gain thus becomes:

$\begin{matrix}{\gamma_{{\hat{k,}t},n} = \left( {\frac{{h_{\hat{k},\hat{k}}}^{2}}{n_{\hat{k},\hat{k}}} + \frac{\sum\limits_{m}\;{\rho_{\hat{k},t,n}{h_{\hat{k},\phi_{m}}}^{2}P_{{\phi_{m,}t},n}}}{n_{\hat{k},\phi_{m}}}} \right)} & (12) \\{{{\max\; C_{\hat{k},t}} = {\sum\limits_{n}\;{\log_{2}\left( {1 + {P_{\hat{k},t,n}\gamma_{\hat{k},t,n}}} \right)}}},{\forall{t \in T}}} & (13)\end{matrix}$

-   -   subject to:

$\begin{matrix}{{{\sum\limits_{n}^{\;}p_{\hat{k},t,n}} \leq P_{k}},{\forall{\hat{k} \in K}},{\forall{t \in T}},} & \left( {14a} \right) \\{\mathcal{L} = {{\sum\limits_{n}^{\;}{\log_{2}\left( {1 + {p_{\hat{k},t,n}\gamma_{\hat{k},t,n}}} \right)}} - {\Omega_{t}\left( {{\sum\limits_{n}^{\;}p_{\hat{k},t,n}} - P_{k}} \right)}}} & (15) \\{{p_{\hat{k},t,n} = {\lambda_{t} - \frac{1}{\gamma_{\hat{k},t,n}}}},{{{if}\mspace{14mu} n_{\hat{k},\hat{k}}} = n_{\hat{k},\phi_{m}}},{p_{\hat{k},t,n}\mspace{14mu}{becomes}\text{:}}} & (16) \\{p_{\hat{k},t,n} = {\lambda_{t} - \frac{n_{\hat{k},\hat{k}}}{{h_{\hat{k},\hat{k}}}^{2} + {\sum\limits_{m}{\rho_{\hat{k},t,n}{h_{\hat{k},\phi_{m}}}^{2}p_{\phi_{m},t,n}}}}}} & (17)\end{matrix}$

Note: Tone/subcarrier spacing is excluded from the optimisation problemsbecause it is a constant and hence the units of the current capacity arebandwidth-normalised (known as bandwidth or spectrum efficiency) in

$\frac{bps}{Hz}.$

An alternative formulation to the above described embodiment allows theexploitation of indirect (phantom/crosstalk) channels over the samespectrum and simultaneously for all or plural existing line users at anyone or more tones, n, subject to a power constraint for the entirespectrum. To illustrate how this is achieved, the problem is decomposed;firstly, the power allocation per tone/carrier is determined and thenthe distribution of tone power between the active users is optimized. Toenable this, the problem becomes:

$\begin{matrix}{\max{\sum\limits_{n}\;{\log_{2}\left( {1 + {P_{n}{\sum\limits_{k}\;\gamma_{k,n}}}} \right)}}} & (18)\end{matrix}$

-   -   subject to:

$\begin{matrix}{{{\sum\limits_{n}^{\;}p_{n}} \leq P_{T}},} & \left( {19a} \right)\end{matrix}$

Applying the Lagrangian:

$\begin{matrix}{\mathcal{L} = {{\sum\limits_{n}\;{\log_{2}\left( {1 + {p_{n}{\sum\limits_{k}\;\gamma_{k,n}}}} \right)}} - {\lambda\left( {{\sum\limits_{n}p_{n}} - P_{T}} \right)}}} & (20)\end{matrix}$

Take

$\frac{\partial\mathcal{L}}{\partial p_{n}}$and then rearrange to obtain:

$\begin{matrix}{{p_{n} = {\lambda^{- 1} - \frac{1}{\sum\limits_{k}\;\gamma_{k,n}}}},} & (21)\end{matrix}$

Equation (21) is substituted into (19a) to calculate the multiplier, λ,and then again into (21) to calculate the optimal spatial frequencypower level.

Now the distribution of p_(n) between K users is optimised.

$\begin{matrix}{\max{\sum\limits_{n}\;{\log_{2}\left( {1 + {p_{k,n}{\sum\limits_{k}\;\gamma_{k,n}}}} \right)}}} & (22)\end{matrix}$

-   -   subject to:

$\begin{matrix}{{{\sum\limits_{k}^{\;}p_{k,n}} \leq p_{n}},} & \left( {23a} \right)\end{matrix}$

Applying the Lagrangian:

$\begin{matrix}{\mathcal{L} = {{\sum\limits_{k}\;{\log_{2}\left( {1 + {p_{k,n}\;\gamma_{k,n}}} \right)}} - {\lambda_{n}\left( {{\sum\limits_{k}p_{k,n}} - p_{n}} \right)}}} & (24)\end{matrix}$

Similarly to previous steps, the optimal power equation is obtained:

$\begin{matrix}{{p_{k,n} = {\lambda_{n}^{- 1} - \frac{1}{\gamma_{k,n}\;}}},} & (25)\end{matrix}$

EXAMPLE-01

Assume two users to share a p_(n). The optimisation problem can besimplified to:max[(1+p_(1,nγ1,n))(1+p_(2,nγ2,n))]  (26)

-   -   subject to:        p _(1,n) +p _(2,n) =p _(n),  (27)

The problem in (26) is easily solvable, two equations and two unknowns.One can prove the optimal power allocation from both problem (22) and(26) is:

$\begin{matrix}{p_{1,n} = \frac{{p_{n}\left( {\prod\limits_{k = 1}^{2}\;\gamma_{k,n}} \right)} + \;\gamma_{1,n} - \;\gamma_{2,n}}{2\;{\prod\limits_{k = 1}^{2}\mspace{11mu}\gamma_{k,n}}}} & (28)\end{matrix}$

Finally p_(2,n) is equal to p_(n)-p_(1,n).

EXAMPLE-02

In terms of signal precoding and real signal injection for a given MPAD(1670) settings, consider the following:

-   -   The data, [d₁ d₂], are first modulated, e.g. using M-QAM, at a        given subcarrier (n) to produce the original data symbols:

$X = \begin{pmatrix}X_{1} \\X_{2}\end{pmatrix}$

-   -   The precoded data symbols (using MICOP-MRC) are calculated as        follows:

$\hat{X} = {{\begin{pmatrix}\frac{h_{1,1}^{*}}{h_{1,1}} & 0 \\0 & \frac{h_{2,2}^{*}}{h_{2,2}} \\\frac{\rho_{1}h_{1,3}^{*}}{h_{1,3}} & \frac{\rho_{2}h_{2,3}^{*}}{h_{2,3}}\end{pmatrix}\begin{pmatrix}X_{1} \\X_{2}\end{pmatrix}} = \begin{pmatrix}\frac{x_{1}h_{1,1}^{*}}{h_{1,1}} \\\frac{x_{2}h_{2,2}^{*}}{h_{2,2}} \\{\frac{x_{1}\rho_{1}h_{1,3}^{*}}{h_{1,3}} + \frac{x_{2}\rho_{2}h_{2,3}^{*}}{h_{2,3}}}\end{pmatrix}}$ where ρ₁ + ρ₂ = 1.${{Note}\mspace{14mu}{that}\mspace{14mu} p_{n}} = {{\frac{x_{1}\rho_{1}h_{1,3}^{*}}{h_{1,3}}}^{2} + {\frac{x_{2}\rho_{2}h_{2,3}^{*}}{h_{2,3}}}^{2}}$where ${\frac{x_{1}\rho_{1}h_{1,3}^{*}}{h_{1,3}}}^{2} = p_{1,n}$ and${{{\frac{x_{2}\rho_{2}h_{2,3}^{*}}{h_{2,3}}}^{2} = {p_{2,n}.{Hence}}},{\rho_{1} = \frac{{h_{1,3}}\sqrt[2]{p_{1,n}}}{{x_{1}h_{1,3}^{*}}}}}\mspace{14mu}$and${\rho_{2} = \frac{{h_{2,3}}\sqrt[2]{p_{2,n}}}{{x_{2}h_{2,3}^{*}}}}\mspace{14mu},$see Example-01. Index n is dropped from the matrices for clarity.

-   -   Non-vectored received signals:

$\overset{\sim}{Y} = {\begin{pmatrix}h_{1,1} & h_{1,2} & h_{1,3} \\h_{2,1} & h_{2,2} & h_{2,3}\end{pmatrix}\begin{pmatrix}\frac{x_{1}h_{1,1}^{*}}{h_{1,1}} \\\frac{x_{2}h_{2,2}^{*}}{h_{2,2}} \\{\frac{x_{1}\rho_{1}h_{1,3}^{*}}{h_{1,3}} + \frac{x_{2}\rho_{2}h_{2,3}^{*}}{h_{2,3}}}\end{pmatrix}}$

-   -   To remove the unwanted coupling after combining, the new channel        coefficients must be calculated using the MRC coefficients since        that the 1630 sees include the MICOP-MRC part of the channel:

${\begin{pmatrix}h_{1,1} & h_{1,2} & h_{1,3} \\h_{2,1} & h_{2,2} & h_{2,3}\end{pmatrix}\begin{pmatrix}\frac{h_{1,1}^{*}}{h_{1,1}} & 0 \\0 & \frac{h_{2,2}^{*}}{h_{2,2}} \\\frac{\rho_{1}h_{1,3}^{*}}{h_{1,3}} & \frac{\rho_{2}h_{2,3}^{*}}{h_{2,3}}\end{pmatrix}} = \begin{pmatrix}{{h_{1,1}} + {\rho_{1}{h_{1,3}}}} & {\frac{h_{1,2}h_{2,2}^{*}}{h_{2,2}} + \frac{\rho_{2}h_{1,3}h_{2,3}^{*}}{h_{2,3}}} \\{\frac{h_{2,1}h_{1,1}^{*}}{h_{1,1}} + \frac{\rho_{1}h_{2,3}h_{1,3}^{*}}{h_{1,3}}} & {{h_{2,2}} + {\rho_{2}{h_{2,3}}}}\end{pmatrix}$

-   -   The vectoring precoder in 1630 becomes:

$\begin{pmatrix}{{h_{1,1}} + {\rho_{1}{h_{1,3}}}} & {\frac{h_{1,2}h_{2,2}^{*}}{h_{2,2}} + \frac{\rho_{2}h_{1,3}h_{2,3}^{*}}{h_{2,3}}} \\{\frac{h_{2,1}h_{1,1}^{*}}{h_{1,1}} + \frac{\rho_{1}h_{2,3}h_{1,3}^{*}}{h_{1,3}}} & {{h_{2,2}} + {\rho_{2}{h_{2,3}}}}\end{pmatrix}^{- 1}$ $\begin{pmatrix}{{h_{1,1}} + {\rho_{1}{h_{1,3}}}} & 0 \\0 & {{h_{2,2}} + {\rho_{2}{h_{2,3}}}}\end{pmatrix}$

Note that the right hand matrix above represents a normalisation toprevent the channel inverse (which is the left hand matrix) fromexcessively amplifying signal components before attempting to transmitthem over the physical channels. A corresponding de-normalisation isthen performed by each receiver. It should be noted that embodiments ofthe present invention are not limited to any particular type ofvectoring or normalisation methodology adopted, but rather can be usedtogether with any appropriate form of vectoring and/or normalisation.However, in the present embodiment, the full system thus becomes:

$\begin{pmatrix}y_{1} \\y_{2}\end{pmatrix} = {\begin{pmatrix}h_{1,1} & h_{1,2} & h_{1,3} \\h_{2,1} & h_{2,2} & h_{2,3}\end{pmatrix}\begin{pmatrix}\frac{h_{1,1}^{*}}{h_{1,1}} & 0 \\0 & \frac{h_{2,2}^{*}}{h_{2,2}} \\\frac{\rho_{1}h_{1,3}^{*}}{h_{1,3}} & \frac{\rho_{2}h_{2,3}^{*}}{h_{2,3}}\end{pmatrix}\begin{pmatrix}{{h_{1,1}} + {\rho_{1}{h_{1,3}}}} & {\frac{h_{1,2}h_{2,2}^{*}}{h_{2,2}} + \frac{\rho_{2}h_{1,3}h_{2,3}^{*}}{h_{2,3}}} \\{\frac{h_{2,1}h_{1,1}^{*}}{h_{1,1}} + \frac{\rho_{1}h_{2,3}h_{1,3}^{*}}{h_{1,3}}} & {{h_{2,2}} + {\rho_{2}{h_{2,3}}}}\end{pmatrix}^{- 1}\begin{pmatrix}{{h_{1,1}} + {\rho_{1}{h_{1,3}}}} & 0 \\0 & {{h_{2,2}} + {\rho_{2}{h_{2,3}}}}\end{pmatrix}{\begin{pmatrix}x_{1} \\x_{2}\end{pmatrix}.}}$

And finally, the transmitted X is estimated at FEQs by:

$\overset{¨}{X} = {\begin{pmatrix}{\overset{¨}{x}}_{1} \\{\overset{¨}{x}}_{2}\end{pmatrix} = {\begin{pmatrix}{{h_{1,1}} + {\rho_{1}{h_{1,3}}}} & 0 \\0 & {{h_{2,2}} + {\rho_{2}{h_{2,3}}}}\end{pmatrix}^{- 1}\begin{pmatrix}y_{1} \\y_{2}\end{pmatrix}}}$

Signal Tracking in 16

-   1. After data source (1611):

$D = \begin{pmatrix}d_{1} \\d_{2}\end{pmatrix}$

-   2. After M-QAM (1621):

$X = \begin{pmatrix}x_{1} \\x_{2}\end{pmatrix}$

-   3. After the vectoring unit (1630):

$\overset{ˇ}{X} = {\begin{pmatrix}{\overset{ˇ}{x}}_{1} \\{\overset{ˇ}{x}}_{2}\end{pmatrix} = {\begin{pmatrix}{{h_{1,1}} + {\rho_{1}{h_{1,3}}}} & {\frac{h_{1,2}h_{2,2}^{*}}{h_{2,2}} + \frac{\rho_{2}h_{1,3}h_{2,3}^{*}}{h_{2,3}}} \\{\frac{h_{2,1}h_{1,1}^{*}}{h_{1,1}} + \frac{\rho_{1}h_{2,3}h_{1,3}^{*}}{h_{1,3}}} & {{h_{2,2}} + {\rho_{2}{h_{2,3}}}}\end{pmatrix}^{- 1}\begin{pmatrix}{{h_{1,1}} + {\rho_{1}{h_{1,3}}}} & 0 \\0 & {{h_{2,2}} + {\rho_{2}{h_{2,3}}}}\end{pmatrix}\begin{pmatrix}x_{1} \\x_{2}\end{pmatrix}}}$

-   -   Expanded to:

$\overset{ˇ}{X} = {\begin{pmatrix}{\overset{ˇ}{x}}_{1} \\{\overset{ˇ}{x}}_{2}\end{pmatrix} = {\frac{1}{\begin{bmatrix}{{\left( {{h_{1,1}} + {\rho_{1}{h_{1,3}}}} \right)*\left( {{h_{2,2}} + {\rho_{2}{h_{2,3}}}} \right)} -} \\{\left( {\frac{h_{1,2}h_{2,2}^{*}}{h_{2,2}} + \frac{\rho_{2}h_{1,3}h_{2,3}^{*}}{h_{2,3}}} \right)*\left( {\frac{h_{2,1}h_{1,1}^{*}}{h_{1,1}} + \frac{\rho_{1}h_{2,3}h_{1,3}^{*}}{h_{1,3}}} \right)}\end{bmatrix}}\begin{pmatrix}{{h_{2,2}} + {\rho_{2}{h_{2,3}}}} & {{- \frac{h_{2,1}h_{1,1}^{*}}{h_{1,1}}} - \frac{\rho_{1}h_{2,3}h_{1,3}^{*}}{h_{1,3}}} \\{{- \frac{h_{1,2}h_{2,2}^{*}}{h_{2,2}}} - \frac{\rho_{2}h_{1,3}h_{2,3}^{*}}{h_{2,3}}} & {{h_{1,1}} + {\rho_{1}{h_{1,3}}}}\end{pmatrix}\begin{pmatrix}{{h_{1,1}} + {\rho_{1}{h_{1,3}}}} & 0 \\0 & {{h_{2,2}} + {\rho_{2}{h_{2,3}}}}\end{pmatrix}\begin{pmatrix}x_{1} \\x_{2}\end{pmatrix}}}$${{and}\mspace{14mu}{hence}},{\overset{ˇ}{X} = {\begin{pmatrix}{\overset{ˇ}{x}}_{1} \\{\overset{ˇ}{x}}_{2}\end{pmatrix} = {\frac{1}{\begin{bmatrix}{{\left( {{h_{1,1}} + {\rho_{1}{h_{1,3}}}} \right)*\left( {{h_{2,2}} + {\rho_{2}{h_{2,3}}}} \right)} -} \\{\left( {\frac{h_{1,2}h_{2,2}^{*}}{h_{2,2}} + \frac{\rho_{2}h_{1,3}h_{2,3}^{*}}{h_{2,3}}} \right)*\left( {\frac{h_{2,1}h_{1,1}^{*}}{h_{1,1}} + \frac{\rho_{1}h_{2,3}h_{1,3}^{*}}{h_{1,3}}} \right)}\end{bmatrix}}\begin{pmatrix}{\left( {{h_{1,1}} + {\rho_{1}{h_{1,3}}}} \right)*\left( {{h_{2,2}} + {\rho_{2}{h_{2,3}}}} \right)} & {\left( {{h_{1,1}} + {\rho_{1}{h_{1,3}}}} \right)*\left( {{- \frac{h_{2,1}h_{1,1}^{*}}{h_{1,1}}} - \frac{\rho_{1}h_{2,3}h_{1,3}^{*}}{h_{1,3}}} \right)} \\{\left( {{h_{2,2}} + {\rho_{2}{h_{2,3}}}} \right)*\left( {{- \frac{h_{1,2}h_{2,2}^{*}}{h_{2,2}}} - \frac{\rho_{2}h_{1,3}h_{2,3}^{*}}{h_{2,3}}} \right)} & {\left( {{h_{2,2}} + {\rho_{2}{h_{2,3}}}} \right)*\left( {{h_{1,1}} + {\rho_{1}{h_{1,3}}}} \right)}\end{pmatrix}\begin{pmatrix}x_{1} \\x_{2}\end{pmatrix}}}}$

-   -   Finally:

$\begin{pmatrix}{\overset{ˇ}{x}}_{1} \\{\overset{ˇ}{x}}_{2}\end{pmatrix} = \begin{pmatrix}\frac{{{x_{1}\left( {{h_{1,1}} + {\rho_{1}{h_{1,3}}}} \right)}*\left( {{h_{2,2}} + {\rho_{2}{h_{2,3}}}} \right)} + {{x_{2}\left( {{h_{1,1}} + {\rho_{1}{h_{1,3}}}} \right)}*\left( {{- \frac{h_{2,1}h_{1,1}^{*}}{h_{1,1}}} - \frac{\rho_{1}h_{2,3}h_{1,3}^{*}}{h_{1,3}}} \right)}}{\left\lbrack {{\left( {{h_{1,1}} + {\rho_{1}{h_{1,3}}}} \right)*\left( {{h_{2,2}} + {\rho_{2}{h_{2,3}}}} \right)} - {\left( {\frac{h_{1,2}h_{2,2}^{*}}{h_{2,2}} + \frac{\rho_{2}h_{1,3}h_{2,3}^{*}}{h_{2,3}}} \right)*\left( {\frac{h_{2,1}h_{1,1}^{*}}{h_{1,1}} + \frac{\rho_{1}h_{2,3}h_{1,3}^{*}}{h_{1,3}}} \right)}} \right\rbrack} \\\frac{{{x_{1}\left( {{h_{2,2}} + {\rho_{2}{h_{2,3}}}} \right)}*\left( {{- \frac{h_{1,2}h_{2,2}^{*}}{h_{2,2}}} - \frac{\rho_{2}h_{1,3}h_{2,3}^{*}}{h_{2,3}}} \right)} + {{x_{2}\left( {{h_{2,2}} + {\rho_{2}{h_{2,3}}}} \right)}*\left( {{h_{1,1}} + {\rho_{1}{h_{1,3}}}} \right)}}{\left\lbrack {{\left( {{h_{1,1}} + {\rho_{1}{h_{1,3}}}} \right)*\left( {{h_{2,2}} + {\rho_{2}{h_{2,3}}}} \right)} - {\left( {\frac{h_{1,2}h_{2,2}^{*}}{h_{2,2}} + \frac{\rho_{2}h_{1,3}h_{2,3}^{*}}{h_{2,3}}} \right)*\left( {\frac{h_{2,1}h_{1,1}^{*}}{h_{1,1}} + \frac{\rho_{1}h_{2,3}h_{1,3}^{*}}{h_{1,3}}} \right)}} \right\rbrack}\end{pmatrix}$

-   4. After MICOP-MRC (1640)

$\hat{X} = {{\begin{pmatrix}\frac{h_{1,1}^{*}}{h_{1,1}} & 0 \\0 & \frac{h_{2,2}^{*}}{h_{2,2}} \\\frac{\rho_{1}h_{1,3}^{*}}{h_{1,3}} & \frac{\rho_{2}h_{2,3}^{*}}{h_{2,3}}\end{pmatrix}\begin{pmatrix}{\overset{ˇ}{x}}_{1} \\{\overset{ˇ}{x}}_{2}\end{pmatrix}} = \begin{pmatrix}\frac{{\overset{ˇ}{x}}_{1}h_{1,1}^{*}}{h_{1,1}} \\\frac{{\overset{ˇ}{x}}_{2}h_{2,2}^{*}}{h_{2,2}} \\{\frac{{\overset{ˇ}{x}}_{1}\rho_{1}h_{1,3}^{*}}{h_{1,3}} + \frac{{\overset{ˇ}{x}}_{2}\rho_{2}h_{2,3}^{*}}{h_{2,3}}}\end{pmatrix}}$

-   -   or equivalently:

$\hat{X} = {\begin{pmatrix}\frac{h_{1,1}^{*}}{h_{1,1}} & 0 \\0 & \frac{h_{2,2}^{*}}{h_{2,2}} \\\frac{\rho_{1}h_{1,3}^{*}}{h_{1,3}} & \frac{\rho_{2}h_{2,3}^{*}}{h_{2,3}}\end{pmatrix}\begin{pmatrix}\frac{{{x_{1}\left( {{h_{1,1}} + {\rho_{1}{h_{1,3}}}} \right)}*\left( {{h_{2,2}} + {\rho_{2}{h_{2,3}}}} \right)} + {{x_{2}\left( {{h_{1,1}} + {\rho_{1}{h_{1,3}}}} \right)}*\left( {{- \frac{h_{2,1}h_{1,1}^{*}}{h_{1,1}}} - \frac{\rho_{1}h_{2,3}h_{1,3}^{*}}{h_{1,3}}} \right)}}{\left\lbrack {{\left( {{h_{1,1}} + {\rho_{1}{h_{1,3}}}} \right)*\left( {{h_{2,2}} + {\rho_{2}{h_{2,3}}}} \right)} - {\left( {\frac{h_{1,2}h_{2,2}^{*}}{h_{2,2}} + \frac{\rho_{2}h_{1,3}h_{2,3}^{*}}{h_{2,3}}} \right)*\left( {\frac{h_{2,1}h_{1,1}^{*}}{h_{1,1}} + \frac{\rho_{1}h_{2,3}h_{1,3}^{*}}{h_{1,3}}} \right)}} \right\rbrack} \\\frac{{{x_{1}\left( {{h_{2,2}} + {\rho_{2}{h_{2,3}}}} \right)}*\left( {{- \frac{h_{1,2}h_{2,2}^{*}}{h_{2,2}}} - \frac{\rho_{2}h_{1,3}h_{2,3}^{*}}{h_{2,3}}} \right)} + {{x_{2}\left( {{h_{2,2}} + {\rho_{2}{h_{2,3}}}} \right)}*\left( {{h_{1,1}} + {\rho_{1}{h_{1,3}}}} \right)}}{\left\lbrack {{\left( {{h_{1,1}} + {\rho_{1}{h_{1,3}}}} \right)*\left( {{h_{2,2}} + {\rho_{2}{h_{2,3}}}} \right)} - {\left( {\frac{h_{1,2}h_{2,2}^{*}}{h_{2,2}} + \frac{\rho_{2}h_{1,3}h_{2,3}^{*}}{h_{2,3}}} \right)*\left( {\frac{h_{2,1}h_{1,1}^{*}}{h_{1,1}} + \frac{\rho_{1}h_{2,3}h_{1,3}^{*}}{h_{1,3}}} \right)}} \right\rbrack}\end{pmatrix}}$

-   5. Finally the transmitted signal Y is modelled as:

$Y = {\begin{pmatrix}y_{1} \\y_{2}\end{pmatrix} = {{\begin{pmatrix}{{h_{1,1}} + {\rho_{1}{h_{1,3}}}} & {\frac{h_{1,2}h_{2,2}^{*}}{h_{2,2}} + \frac{\rho_{2}h_{1,3}h_{2,3}^{*}}{h_{2,3}}} \\{\frac{h_{2,1}h_{1,1}^{*}}{h_{1,1}} + \frac{\rho_{1}h_{2,3}h_{1,3}^{*}}{h_{1,3}}} & {{h_{2,2}} + {\rho_{2}{h_{2,3}}}}\end{pmatrix}\begin{pmatrix}{\hat{x}}_{1} \\{\hat{x}}_{2}\end{pmatrix}} + \begin{pmatrix}n_{1} \\n_{2}\end{pmatrix}}}$

-   -   where n is the background noise.

-   6. At the receiver end, the configuration of the FEQ for a given    line, e.g. k, is (|h_(k,k)|+ρ_(k)|h_(k,3)|)⁻¹.

Generalization of Above Equations of Cover Cases of Multiple CommonIndirect Channels

It will be apparent to a person skilled in the art that the aboveequations may be modified in a straight forward manner to cover morecomplex situations including an arbitrarily large number of user datastreams d₁, d₂, . . . , d_(K), with a (generally) corresponding numberof direct differential mode channels over which to transmitcorresponding streams of QAM constellation points x₁, x₂, . . . , x_(K),an arbitrarily large number of common indirect channels Ψ₁, Ψ₂, . . . ,Ψ_(IDC), where there are IDC indirect channels in total (e.g. made of Mphantom channels φ₁, φ₂, . . . , φ_(M) and IDC-M crosstalk channels). Insuch a case, crosstalk channels can be handled in exactly the same wayas phantom channels in terms of generating and using an extended channelmodel H_(T) as discussed above with particular reference to phantomchannels. Moreover, in such a case, a weighting value can be specifiedfor each combination of an indirect channel and a user data stream, inrespect of each tone, n, giving rise to K×IDC×N weighting values intotal (although a large number of these may be set to 0).

Summary of the Methodology:

It will be apparent to persons skilled in the art from the abovedescription that the method of operation of the system (includingdetermining values for the weighting values) proceeds along thefollowing lines:

-   1. Identify all possible indirect channels for system under    consideration.-   2. Initialise variables/arrays for all possible transfer functions    and weighting values.-   3. For each tone iterate through the following sub-steps:    -   3.1. Identify possible indirect channels (in simple embodiments        exclude all crosstalk differential mode channels for which tone        under consideration is below a threshold associated with the        highest tone that the respective receiver device (associated        with the direct differential mode channel) is operable to        receive data at, and set to 0 all weighting values associated        with the excluded indirect channels).    -   3.2. For each possible indirect channel assess crosstalk        coupling strength and exclude from further consideration all        channels whose crosstalk coupling is below a predetermined        threshold and set weighting values to zero for all thus excluded        indirect channels.    -   3.3. If there are more possible phantom channels at this point        than can be simultaneously (and orthogonally) transmitted onto        by the system, run a phantom selection algorithm to select a        subset of these possible phantoms based on estimations of the        crosstalk couplings and ensuring that the selected phantoms are        orthogonal to one another.    -   3.4. For all remaining indirect channels to be used by the        system, run weighting value determination algorithm to generate        values for the weighting values associated with each remaining        indirect channel, the algorithm using estimations of the        crosstalk couplings between the indirect channels and each used        direct channel (as detected by the associated respective        receiver).    -   3.5. Determine pre-coding coefficients based on the determined        weighting values and estimations of the various channel transfer        function values (including all relevant crosstalk coupling        channel transfer function values). Note that the pre-coding        coefficients calculated in this step include pre-coding        coefficients for pre-coding the signals to be transmitted onto        the direct differential mode channels as well as the indirect        channels.-   4. Operate system using the coefficients calculated in step 3.5 5.    Monitor channel transfer function estimations and repeat step 3 in    respect of any tones for which the estimations change by more than a    predetermined amount.

Note that it is possible to modify step 3.4 to also take into accountlevels of demand for bandwidth from the different receivers/lines whendetermining the weighting values and it is possible to include anadditional step between steps 4 and 5 which monitors the (estimationsof) level of demand and repeats step 3 if necessary based on anassessment of the levels of demand.

Also note that step 1 depends only upon a knowledge of the architectureand capabilities of the system. For example in a system in which commonmodes are never exploited and only first order phantom modes are capableof being exploited, if the system has three twisted pairs, there are 3different possible (non-orthogonal) phantom mode channels and 3different possible indirect crosstalk differential mode channels (onefor each twisted pair).

The amount of change required to trigger a redetermination of theweighting values and thence associated precoding coefficients can betuned to ensure that changes in the system are tracked reasonably wellwithout unduly burdening the system by requiring the large number ofcalculations which the system must perform to be carried out at veryregular intervals (which might be taxing for less powerful processors).

The invention claimed is:
 1. A method of transmitting data from atransmitter device to a first, a second and a third receiver device, thereceiver devices being connected to the transmitter device via a first,a second and a third pair of wires respectively, each receiver devicebeing operable to receive signals detected as a change over time in thepotential difference across the local ends of each respective pair ofwires extending between the receiver and the transmitter devices, thetransmitter device being operable to transmit signals onto the wiresextending between the transmitter device and the receiver devices inorder to transmit signals via the direct differential mode to eachrespective receiver, and is additionally operable to transmit signals toboth receivers via a single common indirect channel, the methodcomprising: measuring the extent of coupling between the common indirectchannel and each of the receiver devices, determining a plurality ofweighting values in dependence upon the measured extent of thecouplings, transmitting a first signal via the direct differential modeover the first pair and a second signal via the direct differential modeover the second pair, and transmitting a combined signal onto theindirect channel, the combined signal comprising a weighted sum of thefirst and second signals, the weighting being done in accordance withthe determined weighting values, and wherein each of the signals isprecoded prior to being transmitted in order to pre-compensate for theexpected effects of cross-talk from the other signals, and wherein thepre-coding of each signal, including the first and the second signal, isperformed in dependence upon the determined weighting values, wherein afirst phantom mode channel is formed using a differential signal betweensignals of the first and second receiver devices, a second phantom modechannel is formed using a differential signal between signals of thefirst and third receiver devices, and a third phantom mode channel isformed using a differential signal between signals of the second andthird receiver devices, and wherein the common indirect channel is asingle ended phantom mode channel selected from a set of possiblephantom mode channels formed by the first, second and third phantom modechannels.
 2. A method according to claim 1 wherein the weighting valuesare determined additionally in dependence upon the instantaneous levelof demand for data to be transmitted to a respective receiver.
 3. Amethod according to claim 2 wherein the transmitter device and thereceiver devices are operating in accordance with a physical layerretransmission scheme whereby a receiver requests retransmission ofreceived data which is irreparably damaged because of errors in thereceived signals or in the detected or recovered data upon receipt.
 4. Amethod according to claim 3 wherein the demand used in determining theweighting values reflects the demand for physical layer re-transmissionof data caused by errors in received data.
 5. A method of transmittingdata from a transmitter device to a first and a second receiver device,the receiver devices being connected to the transmitter device via afirst and a second pair of wires respectively, each receiver devicebeing operable to receive signals detected as a change over time in thepotential difference across the local ends of each respective pair ofwires extending between the receiver and the transmitter devices, thetransmitter device being operable to transmit signals onto the wiresextending between the transmitter device and the receiver devices inorder to transmit signals via the direct differential mode to eachrespective receiver, and is additionally operable to transmit signals toboth receivers via a single common indirect channel, the methodcomprising: measuring the extent of coupling between the common indirectchannel and each of the receiver devices, determining a plurality ofweighting values in dependence upon the measured extent of thecouplings, transmitting a first signal via the direct differential modeover the first pair and a second signal via the direct differential modeover the second pair, and transmitting a combined signal onto theindirect channel, the combined signal comprising a weighted sum of thefirst and second signals, the weighting being done in accordance withthe determined weighting values, and wherein each of the signals isprecoded prior to being transmitted in order to pre-compensate for theexpected effects of cross-talk from the other signals, and wherein thepre-coding of each signal, including the first and the second signal, isperformed in dependence upon the determined weighting values, whereinthe method further comprises monitoring one or more metrics associatedwith the operation of the system and performing a redetermination of atleast some of the weighting values and associated precoding coefficientsin the event that, as a result of the monitoring, one or more monitoredmetric is observed to have changed by more than a predeterminedthreshold amount since last determining or re-determining the weightingvalues.
 6. A method according to claim 5 wherein the monitored metriccomprises one or more of channel transfer function estimations andlevels of demand for data to be transmitted.
 7. A method of transmittingdata from a transmitter device to a first and a second receiver device,the receiver devices being connected to the transmitter device via afirst and a second pair of wires respectively, each receiver devicebeing operable to receive signals detected as a change over time in thepotential difference across the local ends of each respective pair ofwires extending between the receiver and the transmitter devices, thetransmitter device being operable to transmit signals onto the wiresextending between the transmitter device and the receiver devices inorder to transmit signals via the direct differential mode to eachrespective receiver, and is additionally operable to transmit signals toboth receivers via a single common indirect channel, the methodcomprising: measuring the extent of coupling between the common indirectchannel and each of the receiver devices, determining a plurality ofweighting values in dependence upon the measured extent of thecouplings, transmitting a first signal via the direct differential modeover the first pair and a second signal via the direct differential modeover the second pair, and transmitting a combined signal onto theindirect channel, the combined signal comprising a weighted sum of thefirst and second signals, the weighting being done in accordance withthe determined weighting values, and wherein each of the signals isprecoded prior to being transmitted in order to pre-compensate for theexpected effects of cross-talk from the other signals, wherein thepre-coding of each signal, including the first and the second signal, isperformed in dependence upon the determined weighting values, andwherein the manner in which the weighting values are used to affect thefirst and second signals and the manner in which the combined signal isgenerated for transmission over the common indirect channel is performedin such a way that by assigning zero to all of the weighting values fora particular tone, the method reverts to a vectoring technique, for anysuch tone, in which the first and second signals are transmitted onlyonto their respective direct differential mode channels, and thosesignals are pre-coded to pre-compensate for the expected cross-talkeffects of the signal being transmitted onto the other directdifferential mode channel associated with the respective other pair ofwires.
 8. A method according to claim 1 wherein an indirect channelwhich is formed from a direct differential mode of transmission over apair of wires connected between the transmitter device and the thirdreceiver device is used to transmit a combined signal comprising aweighted sum of the first and second signals at a tone which is not usedfor transmissions between the transmitter and the third receiver devicebut is used for transmissions between the transmitter and the first andsecond receiver devices.
 9. A method according to claim 1 wherein thefirst signal is generated in dependence upon: user data to betransmitted to the first receiver device; channel estimations of thefirst direct differential mode channel; channel estimations of theindirect channel between the transmitter and the first receiver devicevia the second direct differential mode channel; channel estimations ofthe extent of mode conversion coupling between the single ended phantomchannel and the first direct differential mode channel as detected bythe first receiver device; and at least some of the weighting values.10. A method according to claim 1 wherein at least some of the weightingvalues take values intermediate between zero and one.
 11. A transmitterdevice for transmitting data from the transmitter device to a first anda second receiver device, the receiver devices being connected, in use,to the transmitter device via a first and a second pair of wiresrespectively, each receiver device being operable to receive signalsdetected as a change over time in the potential difference across thelocal ends of each respective pair of wires extending between therespective receiver device and the transmitter device, the transmitterdevice being operable to transmit signals onto the wires extendingbetween the transmitter device and the receiver devices in order totransmit signals via a direct differential mode to each respectivereceiver device via its respective pair of wires, and is additionallyoperable to transmit signals to both receiver devices via a singlecommon indirect channel, the transmitter comprising: a channel estimatorfor estimating the extent of coupling between the common indirectchannel and each of the receiver devices based on readings received bythe transmitter device from the receiver devices; and a processor fordetermining a plurality of weighting values in dependence upon theestimated extents of the couplings; the transmitter being operable totransmit a first signal via the direct differential mode over the firstpair, to transmit a second signal via the direct differential mode overthe second pair and to transmit a combined signal onto the indirectchannel, wherein the combined signal comprises a weighted sum of thefirst and second signals, the weighting being done in accordance withthe determined weighting values; wherein the transmitter furthercomprises a precoder for precoding the first, second and combinedsignals to pre-compensate them for the expected effects of cross-talkfrom the other ones of these signals, wherein the pre-coding of eachsignal, including the first and the second signals, is performed independence upon the determined weighting values, wherein a first phantommode channel is formed using a differential signal between signals ofthe first and second receiver devices, a second phantom mode channel isformed using a differential signal between signals of the first andthird receiver devices, and a third phantom mode channel is formed usinga differential signal between signals of the second and third receiverdevices, and wherein the common indirect channel is a single endedphantom mode channel selected from a set of possible phantom modechannels formed by the first, second and third phantom mode channels.12. A non-transitory carrier medium carrying computer processorimplementable instructions, which upon execution by the computerprocessor, execute the method of claim
 1. 13. A transmitter device fortransmitting data from the transmitter device to a first and a secondreceiver device, the receiver devices being connected, in use, to thetransmitter device via a first and a second pair of wires respectively,each receiver device being operable to receive signals detected as achange over time in the potential difference across the local ends ofeach respective pair of wires extending between the respective receiverdevice and the transmitter device, the transmitter device being operableto transmit signals onto the wires extending between the transmitterdevice and the receiver devices in order to transmit signals via adirect differential mode to each respective receiver device via itsrespective pair of wires, and is additionally operable to transmitsignals to both receiver devices via a single common indirect channel,the transmitter comprising: a channel estimator for estimating theextent of coupling between the common indirect channel and each of thereceiver devices based on readings received by the transmitter devicefrom the receiver devices; and a processor for determining a pluralityof weighting values in dependence upon the estimated extents of thecouplings; the transmitter being operable to transmit a first signal viathe direct differential mode over the first pair, to transmit a secondsignal via the direct differential mode over the second pair and totransmit a combined signal onto the indirect channel, wherein thecombined signal comprises a weighted sum of the first and secondsignals, the weighting being done in accordance with the determinedweighting values, wherein the transmitter further comprises a precoderfor precoding the first, second and combined signals to pre-compensatethem for the expected effects of cross-talk from the other ones of thesesignals, wherein the pre-coding of each signal, including the first andthe second signals, is performed in dependence upon the determinedweighting values, and wherein the transmitter device is further beingoperable to monitor one or more metrics associated with the operation ofthe system and performing a redetermination of at least some of theweighting values and associated precoding coefficients in the eventthat, as a result of the monitoring, one or more monitored metric isobserved to have changed by more than a predetermined threshold amountsince last determining or re-determining the weighting values.
 14. Thetransmitter device according to claim 13 wherein the monitored metriccomprises one or more of channel transfer function estimations andlevels of demand for data to be transmitted.
 15. A transmitter devicefor transmitting data from the transmitter device to a first and asecond receiver device, the receiver devices being connected, in use, tothe transmitter device via a first and a second pair of wiresrespectively, each receiver device being operable to receive signalsdetected as a change over time in the potential difference across thelocal ends of each respective pair of wires extending between therespective receiver device and the transmitter device, the transmitterdevice being operable to transmit signals onto the wires extendingbetween the transmitter device and the receiver devices in order totransmit signals via a direct differential mode to each respectivereceiver device via its respective pair of wires, and is additionallyoperable to transmit signals to both receiver devices via a singlecommon indirect channel, the transmitter comprising: a channel estimatorfor estimating the extent of coupling between the common indirectchannel and each of the receiver devices based on readings received bythe transmitter device from the receiver devices; and a processor fordetermining a plurality of weighting values in dependence upon theestimated extents of the couplings; the transmitter being operable totransmit a first signal via the direct differential mode over the firstpair, to transmit a second signal via the direct differential mode overthe second pair and to transmit a combined signal onto the indirectchannel, wherein the combined signal comprises a weighted sum of thefirst and second signals, the weighting being done in accordance withthe determined weighting values, wherein the transmitter furthercomprises a precoder for precoding the first, second and combinedsignals to pre-compensate them for the expected effects of cross-talkfrom the other ones of these signals, wherein the pre-coding of eachsignal, including the first and the second signals, is performed independence upon the determined weighting values, and wherein the mannerin which the weighting values are used to affect the first and secondsignals and the manner in which the combined signal is generated fortransmission over the common indirect channel is performed in such a waythat by assigning zero to all of the weighting values for a particulartone, the method reverts to a vectoring technique, for any such tone, inwhich the first and second signals are transmitted only onto theirrespective direct differential mode channels, and those signals arepre-coded to pre-compensate for the expected cross-talk effects of thesignal being transmitted onto the other direct differential mode channelassociated with the respective other pair of wires.